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STOHR'S 


HISTOLOGY 


ARRANGED  UPON  AN  EMBRYOLOGICAL  BASIS 


BY 

DR.   FREDERIC  T.   LEWIS 

ASSISTANT   PROFESSOR   OF   EMBRYOLOGY   AT   THE   HARVARD    JIEDICAL  SCHOOL 


FROM   THE    TWELFTH    GERMAN  EDITION 


DR.   PHILIPP  STOHR 

PROFESSOR    OF   ANATOMY   AT   THE  UNIVERSITY    C'F   WUKZIJUKC; 


Sixtb    Hmerican    BMtion 
limtb  450  IFIlustrations 


PHIL.A.DELPHIA 

P.  BLAKISTON'S    SON    &    CO 

I  O  I  2      W  A  L  X  U  T     S  T  R  E  E  T 
1906 


Copyright,  1903,  by  Dr.  Alfred  Schaper 


Copyright,  1906,  by  Estate  of  Dr.  Alfred  Schaper 


PRESS    or 

WM.    F.    FELL    COMPANY 

1220-24    Sansom    Street 

PH.lAOeLPHIA,     PA. 


NOTE 


In  the  new  edition  of  the  American  translation  of  my 
hand-book  a  number  of  additions  and  changes  have  been 
made  by  the  translator  with  my  permission.  It  is  there- 
fore reasonable  that  I  should  not  take  the  same  responsibihty 
for  the  translation  as  for  the  text  of  the  German  original, 
and  I  would  ask  those  of  my  colleagues  who  wish  to  question 
the  correctness  of  my  assertions  in  their  papers,  to  convince 
themselves,  by  making  comparisons  with  my  last  German 
edition,  that  the  paragraphs  in  question  were  written  by  me. 

Philipp  Stohr. 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 

Open  Knowledge  Commons  (for  the  Medical  Heritage  Library  project) 


http://www.archive.org/details/sthrshistologOOst 


PREFACE 


The  need  of  a  text-book  of  histology  arranged  upon  an  embryological 
basis  has  long  been  felt.  At  the  Harvard  Medical  School  this  need  has 
been  urgent.  There  Professor  Schaper,  the  editor  of  the  five  previous 
American  editions  of  Stohr's  Histology,  planned  such  a  book,  and  after  his 
return  to  Germany  its  preparation  was  begun.  It  is  greatly  to  be  regretted 
that  at  the  time  of  his  death  the  work  was  only  commenced,  for  there  was 
promise  of  a  notable  production. 

When  the  writer  was  informed  that  Professor  Stohr  had  given  gener- 
ous permission  to  adapt  a  new  edition  of  his  Histology  to  American  needs 
it  was  decided  to  rearrange  the  book  upon  an  embryological  plan.  This 
has  been  accompHshed  with  the  loss  of  some  characteristic  features  of  the 
German  edition,  for  which  the  added  material  will,  it  is  hoped,  make  com- 
pensation. Thus  in  order  to  have  space  for  describing  the  controlling  de- 
velopmental features  of  the  organs,  and  for  presenting  their  adult  structure 
somewhat  more  fully,  the  directions  for  preparing  sections  have  been 
reduced  to  the  minimum.  These  may  be  supplemented  by  directions  in 
the  class  room;  and  for  the  small  proportion  of  students  who  intend  to 
practice  elaborate  microscopical  methods,  a  special  text-book  may  be  rec- 
ommended. It  is  not  essential  that  a  physician  should  be  famihar  with  the 
details  of  many  staining  processes,  but  the  structure  of  the  adult  organs  and 
the  developmental  possibiUties  of  their  constituent  tissues  must  be  known. 

The  nomenclature  adopted  is  that  published  by  the  committee  of  the 
German  Association  of  Anatomists  in  1895  (Arch.  f.  Anat.  u.  Phys.;  Anat. 
Ahth.;  Supplement-Band) ,  and  which  is  now  widely  used.  It  is  founded 
upon  the  sound  principle  that  the  name  of  a  structure  should  be  the  simplest 
possible  descriptive  Latin  term  or  phrase.  Since  the  Latin  names  may  be 
translated  into  the  various  modem  languages  the  nomenclature  is  inter- 
national. Moreover  a  large  number  of  the  names  are  commonly  used  in 
their  Latin  forms.  Personal  names  have  been  discarded  (except  Wolffian 
and  Miillerian),  thus  greatly  assisting  the  student.  It  is  obviously 
easier  to  learn  intestinal  glands,  duodenal  glands,  parotid  duct,  etc.,  rather 
than  Lieberkiihn's  glands,  Brunner's  glands,  Stenson's  duct,  and  the  like. 
It  has  been  estimated  that  five  thousand  synon^mis  have  been  rejected  and 
are  to  be  removed  from  the  anatomist's  vocabulary  as  soon  as  possible. 
In  the  following  pages  the  more  common  of  the  rejected  names  have  been 
placed  in  square  brackets,  [  ].     However  difficult  it  may  be  for  the  older 


vi  PREFACE. 

anatomists  to  conform  to  this  nomenclature,  it  seems  clearly  a  duty  to  the 
overworked  medical  students  to  adopt  it. 

Excellent  as  the  German  nomenclature  is,  as  a  whole,  it  is  not  beyond 
improvement,  and  it  may  be  desirable  for  a  committee  of  the  Association 
of  American  Anatomists  to  pubhsh  in  their  Enghsh  forms  a  corresponding 
Hst  of  names.*  As  few  changes  as  possible  should  be  made,  but  it  is  certain, 
for  example,  that  the  ventral  surface  of  the  body  will  not  be  called  anterior , 
or  the  dorsal  surface  posterior.  In  the  following  pages  anterior  always 
means  toward  the  head.  Common  general  terms  should  be  made  even 
more  specific.  For  instance,  it  is  questionable  whether  follicle  (Latin,  a 
small  leather  bag,  a  husk  or  shell)  should  be  apphed  to  anything  other  than 
closed  cysts  hke  the  follicles  of  the  ovar}^  and  thyreoid  gland.  Its  ap- 
phcation  by  the  Germans  to  the  sheath  of  the  hair  and  by  many  Amer- 
icans to  soUd  nodules  of  lymphoid  tissue  may  lead  the  student  to  wonder  if 
"follicle"  is  not  a  colloquial  rather  than  a  scientific  term. 

The  attention  of  all  students  should  be  called  to  the  American  Journal 
0}  Anatomy,  the  quarterly  publication  of  the  Association  of  American 
Anatomists,  which  contains  the  results  of  current  American  anatomical 
and  histological  investigations.  It  probably  affords  the  most  satisfactory 
means  by  which  a  physician  may  keep  in  touch  with  these  sciences. 

The  writer  has  many  acknowledgments  to  make  for  help  received. 
Messrs.  P.  Blakiston's  Son  &  Co.,  and  Mr.  William  T.  Ohver,  the  artist 
who  has  drawn  the  more  elaborate  of  the  new  figures,  have  rendered  all  the 
assistance  possible.  Members  of  several  departments  at  the  Harvard 
Medical  School  have  given  valuable  advice,  and  Dr.  G.  H.  Wright,  As- 
sistant in  Dental  Histology,  has  arranged  a  considerable  portion  of  the 
section  on  the  teeth.  It  is  a  privilege  to  present  for  the  first  time  in  a  text- 
book, the  discoveries  of  Dr.  James  H.  Wright  regarding  the  origin  of  blood 
plates.  His  remarkable  conclusion  that  they  are  fragments  of  pseudopodia 
of  the  giant  cells  seems  established  beyond  doubt  by  an  examination  of 
his  specimens. 

Finally  it  is  a  pleasure  to  record  that  after  studying  iiistology  and  em- 
br\'ology  under  Professor  Charles  S.  Minot,  the  writer  has  for  several 
years  enjoyed  the  closest  association  with  him  in  his  scientific  work.  The 
results  of  such  unusual  privilege  should  be  found  reflected  in  this  edition 
of  Professor  Stohr's  Histology. 

Frederic  T.  Lewis. 

Cambridge,  Massachusetts, 
September,  1906. 

*  The  writer  has  since  been  informed  that  Messrs.  Blakiston's  Son  &  Co.  have  in  press 
such  a  list  prepared  by  Professor  Barker  and  entitled  "Anatomical  TerminoIog>-."  The 
orderly  arrangement  of  these  descriptive  names  makes  the  Latin  list — and  undoubtedly 
their  English  version  also — an  excellent  means  by  which  students  may  review  anatomy. 


CONTENTS 


PART  I. 


MICROSCOPIC  ANATOMY. 


I.    CYTOLOGY. 

PAGE  PAGK 

The  Cell, i      Vital  Phenomena, 7 

Protoplasm.  Amoeboid  Motion. 

Nucleus.  Formation    and     Reproduction    of 

Centrosome.  Cells, 9 

Cell  Wall.  Mitosis. 

Form  and  Size  of  Cells, 7              Amitosis. 

Cytomorphosis, 15 

II.  GENERAL  HISTOLOGY. 

Histogenesis, 18  J  Nerve  Tissue, 90 

Segm.entation  and  the  Formation  of  Development  of, — 

the  Germ  Layers.  i  The  central  tract. 

The    Fundamental    Tissues.  The  spinal  ganglia. 

^  ,  The  ventral  roots. 

Epithelia, 26  „,  ^,     . 

.  i  he  sympathetic  svstem. 

Origin.  „,  ,      , 

°  The  cerebral  nerves, 

bhapes  of  Epithelial  Cells.  o^       ^  r 

^  ^  Structure  of, — 

Number  of  Layers.  t-t         ^,  , 

■'  Nerve  libers  and  nerves. 

Differentiation.  „  ,. 

bensory  endings. 
Processes  of  Secretion.  Ti^r  .  ,- 

Motor  endings. 
The  Nature   and   Classification   of  „        .. 

Ganglia. 

Glands.  t-u        •     1        j 

ihe  spinal  cord. 

Mesenchymal  Tissues, 38 

Reticular  Tissue.  Vascular  Tissue, ,24 

Mucous  Tissue.  Bloodvessels. 

Connective  Tissue.  Development. 

Tendon.  Capillaries. 

Cartilage.  Arteries. 

Bone.  Veins. 

Joints.  The  heart. 

Teeth    (including    the  Ectodermal                       Lymphatic  Vessels. 

Enamel  Organs).  ^^°°'^- 

Red  corpuscles. 

Muscle  TisstJE, 77  White  corpuscles. 

Smooth  Muscle.  Blood  plates. 

Cardiac  Muscle.  Plasma. 

Striated  Muscle.  Lymph. 

vii 


VUl 


CONTENTS. 


III.    SPECIAL  HISTOLOGY. 


Blood  Forming  and  Blood  Destroy- 
ing Org.\ns, 15- 

Bone  Marrow. 

Lymph  Nodules  and  Lymph  Glands. 

Haemolymph  Glands. 

Spleen. 

The  Entodermal  Tract, 1 65 

The  Mouth  and  Pharj-nx, r65 

Development. 

Palatine  tonsils. 

Thymus. 

Thyreoid  gland. 

Parathyreoid  glands. 

Glomus  caroticum. 

Tongue. 

Oral  and  pharyngeal  cavities. 

Glands  of  the  oral  cavity. 
The  Digestive  Tube 193 

Development. 

Oesophagus. 

Stomach. 

Small  Intestine. 

Large  Intestine. 

Rectum  and  Anus. 

The  Liver, 218 

The  Pancreas, 2?o 

The  Respiratory  Tract, 234 

Development. 

Lar}'n.x. 

Trachea,  Bronchi. 

Lungs. 


Urinary  Org.^ns, 244 

Wolffian  Body. 

Pronephros. 

Kidney. 

Renal  pelvis  and  ureter. 

Bladder. 

Urethra  (in  the  female). 

Male  Genital  Organs 263 

Development. 

Testis. 

Epididymis. 

Ductus  deferens. 

Seminal  Vesicles  and  Ejaculatory 

Ducts. 
Appendices  and  Paradidymis. 
Prostate. 
Urethra  and  Penis. 


Female  Genital  Organs, 285 

Development. 
Ovary. 
Epoophoron. 
Uterine  Tubes. 
Uterus. 

Menstruation. 

Development   of   the   decidual 

membranes. 
Structure    of    the    membranes 

and  placenta. 
Umbilical  Cord. 
Vagina  and  External  Genital   Or- 
gans. 

Skin, .V^ 

Nails. 

Hair. 

Sebaceous  glands. 

Sweat  glands. 

Mammary  glands. 

Sltrarenal  Glands, ;^  ^  • 

Brain  .\nd  Sense  Organs, 3^4 

Brain, 3'1 

Development. 

Medulla  oblongata. 

Pons. 

Cerebellum. 

Hemispheres. 

Hypophysis. 

Pineal  body. 

Meninges. 
Eye, .v^  .5 

Development. 

Retina. 

Optic  nen^e. 

Lens. 

Vitreous  body. 

Tunica  vasculosa. 

Tunica  fibrosa. 

Vessels,  chambers,  and  nerves. 

Eyelids. 

Lachrymal  glands. 
Ear, 37'*^ 

Development. 

Internal  ear, — 

Sacculus. 

Utriculus. 

Semicircular  ducts,  and 


CONTEXTS. 


IX 


Brain  and  Sense  Organs — Continued.  page       Brain  and  Sense  Organs — Cotiliniied.      page 

Cochlea.  Xose, 395 

Middle  ear.  Respiratory  region. 

External  ear.  Olfactory  region. 


PART  II. 

THE  PREPARATION  AiND  EXAMINATION  OF  MICRO 
SCOPICAL  SPECIMENS. 


Fresh  Tissues, 400  ;   Staining  and  MorNxiNG, . 


Isolation. 

Sectioning  Fresh  Material. 

Fixation. 

Decalcification. 

Imbedding. 


General  Stains. 
Special  Stains. 
The  Microscope. 
Drat;\tngs. 

Reconstructions. 


PART  I. 

MICROSCOPIC  ANATOMY. 


I.  CYTOLOGY. 

THE  CELL. 


Since  1839  it  has  been  kno-^n  that  all  plants  and  animals  are  composed 
of  small  structural  elements  called  cells  (Latin,  cellula;  Greek,  k'j-oq). 
The  lowest  forms  of  animals  and  of  plants  are  ahke  in  being  single  cells 
throughout  life.  The  more  complex  organisms  are  groups  of  cells  which 
have  been  derived,  by  process  of  repeated  division,  from  a  single  cell, 
the  fertilized  o^aim.  Thus  the  human  body,  which  begins  as  one  cell, 
becomes  in  the  adult  an  aggregation  of  cells  variously  modified  and  adapted 
to  special  functions.  Since  the  liver  is  a  mass  of  essentially  similar  cells, 
the  problems  of  its  functional  activity  are  the  problems  of  the  functions 
of  a  single  one  of  its  cells.  The  diseases  of  the  liver  are  the  result  of 
changes  occurring  in  these  cells,  which  must  be  restored  to  a  normal  con- 
dition to  effect  a  cure.  As  this  is  equally  true  of  other  organs,  it  is  evident 
that  cytology,  the  science  of  cells,  is  a  basis  for  both  physiology  and 
pathology. 

A  cell  may  be  defined  as  a  structural  element  of  limited  dimensions 
which  under  certain  conditions  can  perform  the  functions  of  assimilation, 
growth,  and  reproduction.  Because  of  these  possibihties  a  cell  may  be 
considered  an  elementary  organism.  It  is  described  as  a  mass  of  proto- 
plasm containing  a  nucleus.  A  third  element,  the  centrosome,  is  found 
in  the  cells  of  animals,  but  not  in  those  of  the  higher  plants.  The  centro- 
some becomes  prominent  when  a  cell  is  about  to  divide.  At  other  times, 
in  many  kinds  of  cells,  it  has  been  found  as  a  minute  granule  which  may 
be  in  the  center  of  a  very  small  clear  spot  in  the  protoplasm.  Ordinarily 
it  cannot  be  seen  unless  cell  division  is  about  to  occur.  Some  authorities 
regard  the  centrosome  as  a  temporary  structure  which  forms  shortly 
before   division   begins   and   disappears   after  it   is   completed.     Others 


HISTOLOGY. 


regard  it  as  a  permanent  and  essential  part  of  a  cell,  which  accordingly 
consists  of  protoplasm,  nucleus,  and  centrosome. 

PROTOPLASM. 

Protoplasm  is  the  Hving  substance  of  which  cells  are  composed. 
More  specifically  the  term  is  applied  to  this  Hving  substance  exclusive  of 
the  nucleus,  or  to  the  corresponding  dead  niaterial,  provided  that  death 
has  not  changed  its  physical  properties.  It  has  been  proposed  to  substitute 
the  name  cytoplasm  for  protoplasm  in  the  restricted  and  earher  sense  of 
the  term,  to  call  the  nuclear  substance  nucleoplasm  [karyoplasm],  and 
to  consider  both  cytoplasm  and  nucleoplasm  as  varieties  of  .protoplasm. 
Although  these  names  are  often  employed,  the  cell  substance  apart  from 
the  nucleus  is  ordinarily  called  protoplasm. 


Achromatic 
substances 
of  the  nu- 
cleus. 


^Nuclear  membrane. 

[Ground    sub-        fe^j^-^'i^^^Wfe^       '      ' 
\   stance.  '  -M.W^^^^'^^'^*^ 

Jfty 

Exoplasm.  .  _J,-c!i\-V'2'! 


Interfilar  mass. 


„  f  Diplosomc  in 

_     Centrosome.-^    archoplasm. 


Chromatic 
substances 
of  the   nu- 
cleus. 


T^r-  h  -  Nucleolus. 


Microsomes. 


Cell  membrane. 


Fig.  I.— Diagram  of  a  Ckul.     Microsomes  and  filar  mass  only  partly  sketched. 

Protoplasm  is  a  heterogeneous  mixture  of  substances  forming  a  soft, 
viscid  mass  of  neutral  reaction.  In  distilled  water  it  swells  but  does  not 
disappear.  It  consists  of  water,  salts  and  organic  substances,  some  in 
solution,  and  some  in  a  colloidal  state.  The  organic  bodies  are  classed  as 
proteids,  glycogen  or  some  alUed  carbohydrates,  and  hpoid  (fat-Uke) 
bodies.     Protoplasm  may  exist  in  a  numberless  variety  of  forms. 

On  microscopic  examination  protoplasm  is  seen  to  contain  small 
granules,  microsomes.  In  different  cells  these  vary  in  abundance  and  in 
character.  They  may  be  absent  from  the  outer  layer  of  protoplasm, 
the  exoplasm,  which  is  firmer  and  chemically  different  from  the  inner  endo- 
plasm,  and  perhaps  has  a  separate  function.     The  microsomes  have  been 


PROTOPLASM. 


considered  both  as  inert  bodies  and  as  the  essential  Hving  basis  of  proto- 
plasm. The  simplest  description  of  protoplasmic  structure  is  that  it 
consists  of  a  fluid  ground  substance  in  which  microsomes  are  embedded. 

With  high  magnification  it  appears  that  the  protoplasm  contains  a 
network  of  filaments  (called  mitome,  or  the  filar  mass,  from  the  Greek 
ficzoc  and  Latin  filum,  both  meaning  "a  thread," — spongioplasm  is 
another  synonym).  This  network  is  embedded  in  a  more  or  less  homo- 
geneous and  chemically  different  ground  substance  (paramitome,  interfilar 
mass,  or  hyaloplasm).  Some  of  the  filaments  appear  as  rows  of  micro- 
somes, but  small  particles  may  also  be  found  in  the  ground  substance 
between  the  filaments.  The  conception  of  protoplasm  as  fibrillar  or 
reticular  has  been  considered  at  variance  with  the  "granular  theory," 
yet  undoubtedly  both  fibrils  and  granules  occur  in  protoplasm. 

According  to  a  third  interpretation  protoplasm  has  the  structure  of 
foam,  or  of  an  emulsion, — that  is,  it  consists  of  minute  droplets  of  one 
substance  completely  surrounded  by  walls  of  a  dift'erent  substance.  This 
view,  which  has  much  in  its  favor,  is  not  inconsistent  with  the  presence 
of  granules  or  of  fibrils  scattered  through  the  mass. 

In  addition  to  these  general  char- 
acteristics the  protoplasm  of  particular 
ceUs  may  contain  other  structures  of 
various  significance.  These  may  be 
grouped  as  follows: 

I.  Fibrils.  Although  an  obscure 
fibrillar  network  may  be  characteristic 
of  all  cells,  a  high  development  and 
orderly  arrangement  of  fibrils  occurs 
only  in  certain  specialized  cells,  as  for 

example,  in  muscle,  ner^^e,  and  connective  tis- 
sue cells.  These  fibrils  are  of  very  difl'erent 
sorts  and  will  be  described  more  fully  in  the 
section  on  General  Histology. 

2.  Granules.  These  are  not  the  micro- 
somes found  in  all  protoplasm,  but  are  larger 
bodies  of  definite  staining  reaction,  which  often 
are  important  secretory  products  elaborated  by 
the  cell.  In  many  gland  cells,  and  in  the 
"granular"  white  blood  corpuscles,  these 
structures  are  conspicuous.  Other  granules 
may  be  excretory  or  waste  products  of  the  ceU, 
and  some  of  these,  which,  without  being 
stained,  are  deeply  colored,  are  called  pigment  granules. 


Fig.  2. — Fibrils  in  a  Nerve  Cell. 


Nissl's  bodies. 


Fig 


.  3. — Clumps  of  Granules 
(Nissl's  Bodies)  in  a  Nerve 
Cell. 


HISTOLOGY. 


■^   '^^i'. 


L;^ 


Fig.  4.— \'acioles  in  Young 
Fat  cells. 


Nucleus. 


/ 


Capsule.       Canaliculi. 

Fig.  5.— Canals  (Tropho- 
SPONGiUM)  IN  A  Nerve 
Cell. 


3.  Vacuoles.  Well  defined,  round  spaces, 
apparently  empty,  may  occur  in  the  protoplasm 
due  to  the  formation  of  droplets  of  fat  or  of  watery 
fluids.  They  vary  greatly  in  size,  and  one  or  sev- 
eral of  these  vacuoles  may  be  found  in  a  single  cell. 

4.  Canals,  of  two  sorts,  (a)  Secretory  canals, 
which  occur  in  protoplasm  of  gland  cells  and 
empty  into  the  gland  cavity  or  lumen;  (b)  fine 
tubes  which  communicate  with  lymphatic  spaces 
outside  of  the  cell.  They  are  found  in  all  cells  of 
higher  physiological  importance,  but  are  lacking 
in  most  of  the  supporting  tissues  and  in  stratified 
epithelium.  They  presumably  share  in  nourish- 
ing the  cell  and  have  been  called  the  "tropho- 
spongium."  This  name,  implying  a  network, 
is  due  to  the  opinion,  not  estabhshed  however, 
that  the  Httle  canals  are  occupied  by  cell  pro- 
cesses extending  into  the  protoplasm  from  adjoin- 
ing "capsule  cells."  Other  investigators  consider 
that  the  trophospongium  canals  are  wholly  within 
the  cell  and  constitute  a  form  of  vacuole. 

5.  Closed  networks,  which  do  not  open  at 
the  periphery  of  the  cell.  This  "reticular  appa- 
ratus" has  been  found  in  nerve,  cartilage  and 
many  gland  cells.     Its  significance  is  unknown. 

6.  Inclusions.  These  are  foreign  bodies 
which  have  been  ingested  by  the  cell  and  are 
found  in  the  protoplasm.  Inert  crystalloid  sub- 
stances formed  within  the  cell  are  also  called 
inclusions.  The  name  "paranucleus"  has  been 
apphed  to  various  structures,  such  as  a  dead  cell 
ingested  by  a  living  one,  a  transformation  of  the 
centrosome,  or  a  mass  of  secretion.  Some  of  the 
paranuclei  are  still  obscure. 

NUCLEUS. 

The  nucleus  (Latin,  m<c/m5,  "the  kernel  of 

Fig.    7.— Crystalloid    Inclu-  ,,      /-,         1  u  ^)i\   •  ii     1    n       j 

sioNS.  Interstitial  Cells     a  nut     ,    Greek,  Kapuov,      a   nut     )  IS  a  WCll   Clermeci, 
OF  THE  Testis.  ,  .  j        •  1  • 

refractive  body  of  vesicular  form  situated  within 
the  cell.  It  consists  of  a  membrane  enclosing  a  mass  of  ground  substance, 
or  nuclear  sap,  in  which  there  is  a  fibrillar  network  associated  with  some 


Reticular  apparatu.s. 

Fig.  6.— Rkticl-lar  Network 
IN  A  Nerve  Cell.  (After 
Golgi.) 


NUCLEUS.  5 

deeply  staining  bodies.  The  ground  substance  and  network  are  closely 
related  to  the  corresponding  structures  in  the  protoplasm.  In  fact,  at  the 
time  of  cell  division,  when  the  nuclear  membrane  disappears,  the  network 
and  ground  substance  of  nucleus  and  protoplasm  are  respectively  con- 
tinuous -^-ith  one  another.  The  ground  substance  and  a  dehcate,  fibrillar 
portion  of  the  network  do  not  stain  readily;  therefore  they  are  called 
achromatic  substances.  The  fine  achromatic  fibrils  of  the  network  are 
further  designated  as  linin  fibrils.  Since  the  linin  fibrils  cannot  be  isolated 
for  chemical  analysis,  their  composition  is  unkno\^^l.  There  are  two 
deeply  staining  or  chromatic  substances  found  in  the  nucleus.  One 
of  these,  chromatin,  is  its  most  essential  and  characteristic  element.  Chemi- 
cally it  is  a  nucleo-proteid,  but  undoubtedly  it  exists  in  several  varieties. 
A  portion  which  responds  to  acid  dyes  is  called  oxychromatin,  in  distinc- 
tion from  the  ordinary  form  which  takes  the  basic  stains.  Chromatin  is 
distributed  as  irregular  granules  or  coarse  strands  along  the  linin  fibrils, 
thus  tending  to  form  a  network  (Fig.  i,  and  Fig.  i8,  p.  i6).  Often  a 
nucleus  presents  from  one  to  several  large  clumps  of  chromatin,  kno^^n 
as  chromatin  "knots."  These  are  to  be  distinguished  from  the  round 
masses  of  pyrenin,  called  nucleoli,  which  are  found  between  the  meshes 
of  the  nuclear  network.  All  nuclei  contain  chromatin,  but  many  are 
without  nucleoli.  The  latter  are  present  with  great  regularity  in  certain 
kinds  of  cells.  Usually  only  one  ia  found  in  a  nucleus,  although  several 
may  occur  (Fig.  i8).  They  differ  from  chromatin  chemically,  as  is 
e\ddent  from  differences  in  staining,  and  also  functionally,  as  is  seen  during 
cell  division.  Pyrenin,  of  which  the  nucleolus  is  said  to  be  composed,  is, 
how'ever,  a  cytological  rather  than  a  chemical  term. 

The  nuclear  membrane  is  usually  described  as  formed  of  amphipy- 
renin,  a  term  of  questionable  value.  The  membrane  may  consist  of  a  dis- 
tinct chemical  substance  as  the  name  suggests,  or  it  may  be  rather  a  conden- 
sation of  the  nuclear  reticulum,  in  which  the  linin  fibrils  terminate.  A  nuclear 
membrane  may  be  simulated  by  a  thin  superficial  layer  of  chromatin. 

Every  cell  contains  a  nucleus  consisting,  as  has  been  shown,  of  nuclear 

membrane,  ground  substance,  a  network  of  Hnin  fibrils  and  of  chromatin, 

with  perhaps  a  nucleolus.     Non-nucleated  bodies  Hke  the  mammahan 

red  blood  corpuscles,  and  the  dead  outer  cells  of  the  skin  have  lost  their 

nuclei  in  the  course  of  development.     Occasionally  a  single  cell  contains 

two  nuclei,  as  is  frequent  in  the  liver,  or  even  several  nuclei,  as  in  certain 

bone  cells. 

CENTROSOME. 

The  centrosome  is  a  minute  body  consisting  of  a  homogeneous  or 

sometimes  reticular  mass,  the  centroplasm,  which  contains  a  much  smaller 


6  HISTOLOGY. 

body,  the  centriole.  Such  centrosomes  have  been  observed  in  the  in- 
vertebrate animals.  The  cells  of  vertebrates  are  not  regarded  as  favorable 
for  investigations  of  the  finer  structure  of  centrosomes.  In  them  generally 
both  centroplasm  and  centriole  appear  as  a  single  small  granule,  the 
centrosome.  This  granule  is  usually,  but  not  always,  surrounded  by  a 
zone  of  protoplasm  which  is  so  modified  as  to  form  a  darker  or  a  lighter 
area,  the  archo plasm  (Fig.  i).  (The  archoplasm  of  certain  spermatic  cells 
is  called  the  idiozome.)  The  centrosome  may  be  near  the  nucleus  or 
distant  from  it,  frequently  being  found  between  the  nucleus  and  the  free 
surface  of  the  cell.  Rarely,  as  in  a  few  invertebrates  and  in  cancer  cells,  ^ 
the  centrosome  has  been  found  within  the  nucleus.  In  many  gland  cells 
it  lies  where  the  secretion  accumulates,  the  expulsion  of  wliich  is  accom- 
plished by  the  contraction  of  the  protoplasmic  framework  between  the 
masses  of  secretion.  In  the  intestinal  epithehal  cells  which  send  out 
motile  projections  of  protoplasm  (pseudopodia),  the  centrosome  lies  just 
beneath  the  place  of  origin  of  these  projections.  If  one  considers  also  the 
relation  of  the  centrosome  in  the  spermatozoa  as  well  as  its  role  in  cell 
division,  it  seems  almost  certain  that  the  centrosome  is  the  active  or  passive 
center  of  the  motor  functions.  In  connection  with  cell  division,  the 
centrosome  undergoes  a  cycle  of  changes  of  varying  duration.  That 
stage  which  is  continued  longest  is  characterized  by  a  doubling  of  the 
centrosome,  following  the  division  of  the  centriole  in  two.  The  double 
body  thus  formed  is  the  diplosome.  In  many  resting  cells,  or  those  not 
actually  in  the  process  of  division,  a  diplosome  is  found,  and  this  is  signifi- 
cant as  indicating  the  readiness  of  the  cell  for  undergoing  division  without 
delay. 

CELL  WALL. 

A  cell  wall  or  cell  membrane  is  an  independent  membranous  layer 
covering  a  cell  and  being  clearly  distinct  from  the  underlying  protoplasm. 
It  is  not  an  essential  constituent  of  a  cell.  Often  it  is  lacking,  and  when 
present  it  is  either  a  modification  or  a  secretion  of  the  peripheral  protoplasm. 
If  the  membrane  surrounds  the  cell  on  all  sides  it  is  called  a  pellicula; 
if  it  is  on  only  one  side,  covering  the  free  surface,  it  is  a  cuticula.  (The 
former  term  is  seldom  used.)  Cells  may  unite  with  one  another  by  proto- 
plasmic processes  of  varying  length  and  width,  thus  forming  cellular 
networks;  or  they  may  completely  fuse  so  that  their  nuclei  appear  irregu- 
larly distributed  through  a  single  mass  of  protoplasm.  Such  a  formation 
is  a  syncytium  [plasmodium].  This  name  is  apphed  also  to  such  structures 
as  the  striated  muscle  fiber,  due  not  to  the  fusion  of  cells  but  to  the  multi- 
plication of  nuclei  in  an  undivided  mass  of  protoplasm. 


FORM  AND   SIZE   OF   CELLS.  7 

Although  cell  membranes  are  usually  lacking,  or  if  present  are  often  in- 
conspicuous in  animal  ceUs,  they  are  highly  developed  in  plants.  Thus  cork 
is  a  mass  of  dead  cells  from  which  nuclei  and  protoplasm  have  disappeared, 
leaving  only  the  cell  walls.  In  describing  cork,  Robert  Hooke  introduced  the 
name  "cell,"  in  1667.  He  wrote:  "I  took  a  good  clear  piece  of  Cork  and  with  a 
Pen-knife  sharpened  as  keen  as  a  razor,  I  cut  a  piece  of  it  off  and  thereby  left 
the  surface  of  it  exceedingly  smooth,  then  examining  it  very  diligently  with  a 
microscope,  me  thought  I  could  perceive  it  to  be  a  Httle  porous.     .     .     .     These 

pores  or  cells  were  not  very  deep  but  consisted  of  a  great  many  Httle  Boxes -" 

In  this  way  one  of  the  briefest  and  most  important  of  scientiiic  terms  was  intro- 
duced. 

FORM  AND  SIZE  OF  CELLS. 

Cells  are  regarded  as  typically  spherical  in  form.  Spherical  cells 
are  comparatively  numerous  in  the  embryo,  and  in  the  adult  the  resting 
white  blood  corpuscles  which  float  freely  in  the  body  fluids  assume  this 
form.  Such  cells  are  circular  in  cross  section.  When  spherical  cells 
are  subjected  to  the  pressure  of  similar  neighboring  cells  they  become 
polyhedral  and  usually  -appear  six-sided  in  cross  section.  Such  cells,  as 
a  whole,  may  be  cuboidal,  columnar,  or  flat.  Certain  cells  become  fusi- 
form (spindle-shaped)  or  are  further  elongated  so  as  to  form  fibers;  others 
send  out  radiating  processes  and  are  called  stellate.  Thus  the  form  of 
cells  is  extremely  varied.  The  shape  of  the  nucleus  tends  to  correspond 
with  that  of  its  cell.  It  is  usually  an  elliptical  body  in  elongated  cells, 
and  spherical  in  round  or  cuboidal  cells. .  In  stellate  cells  it  is  either 
spherical  or  somewhat  elongated.  Crescentic  nuclei  and  others  more 
deeply  and  irregularly  lobed  are  found  in  some  of  the  white  blood  corpuscles 
and  in  giant  ceUs. 

The  size  of  cells  ranges  from  that  of  the  yolks  of  birds'  eggs — 
which  are  single  cells  at  least  shortly  before  being  laid — down  to  micro- 
scopic structures  four  thousandths  of  a  milHmeter  in  diameter.  The 
thousandth  of  a  millimeter  is  the  unit  employed  in  microscopic  measure- 
ments. It  is  called  a  micron,  and  its  symbol  is  the  Greek  letter  u.  The 
small  cells  referred  to  are  therefore  four  microns,  4  /^,  in  diameter.  The 
size  of  any  structure  in  a  section  of  human  tissue  may  be  roughly  estimated 
by  comparing  its  dimensions  with  the  diameter  of  a  red  blood  corpuscle 
found  in  the  same  section.  These  red  corpuscles  are  quite  uniformly 
7.5  /y.  in  diameter. 

VITAL  PHENOMENA. 

The  vital  properties  of  cells  are  more  fully  treated  in  text-books  of 
physiology.  They  include  the  phenomena  of  irritability,  metabolism, 
contractility,    conductivity,    and    reproduction.     Under   irritabiUty    may 


8  HISTOLOGY. 

be  grouped  the  response  of  cells  to  stimuli  of  various  sorts  such  as  heat, 
light,  electricity,  chemical  reagents,  the  nervous  impulse,  or  mechanical 
interference.  Metabolism,  in  a  wide  sense,  includes  the  ingestion  and  as- 
similation of  food,  the  elaboration  and  secretion  of  desirable  products, 
together  with  the  elimination  of  waste  products.  ContractiUty  may 
be  manifest  in  the  locomotion  of  the  entire  cell,  in  the  vibratile  action  of 
slender,  hair-like  processes,  the  cilia,  or  in  contraction  of  the  cell  body. 
Conductivity  is  the  power  of  conveying  impulses  from  one  part  of  the  cell 
to  another.  Reproduction  is  seen  in  the  process  of  cell  division.  Many 
phases  of  these  activities  are  observed  in  microscopic  sections  and  as  such 
they  will  be  referred  to  in  later  chapters.  A  few  which  are  of  general 
occurrence  will  be  described  presently. 

AMOEBOID  MOTION. 

The  unicellular  animal.  Amoeba,  exhibits  a  type  of  motility  known 
as  amoeboid,  which  has  been  observed  in  many  sorts  of  cells  in  the  verte- 
brate body.     In  marked  cases,  as 
0  1/,        12         2'  i^    certain  white   blood  corpuscles 

^^        W$     04   '■^^     «^^  (the  leucocytes),  the  cell  protoplasm 

'«>      ^^"      -^     W^     fe*^  sends   out  fine  or  coarse  processes 

which  divide  or  fuse  with  one  an- 
other, causing  the  cell  to  assume  a 
great  variety  of  forms.  The  pro- 
cesses may  be  retracted,  or  thev  may 

Fig.  8.— Leucocytes  of  a  Frog.    X  560.  " 

Changes   in  form   observed  during  ten   minutes;        bcCOmC     attached      SOmCwhcre     and 
o,  at  the  beginning  of  the  observation;    '2,  a.  ,  ,,  .      ,  ^    ,,  hit 

half  minute  later,  etc.  draw  thc  remainder  of  the  cell  body 

after  them,  the  result  of  which  is 
locomotion  or  the  so-called  wandering  of  the  cell.  Such  wandering  cells 
play  an  important  part  in  the  economy  of  the  animal  body.  Their  proc- 
esses can  flow  around  granules  or  cells  and  thus  enclose  them  in  proto- 
plasm. Some  of  these  ingested  bodies  may  be  assimilated  by  the  cell  as 
a  result  of  complex  chemical  and  osmotic  reactions.  Cells  which  feed  on 
foreign  particles  and  can  alter  or  digest  them  are  known  as  phagocytes. 
Amoeboid  movements  take  place  very  slowly.  In  preparations  from  warm- 
blooded animals  they  may  be  accelerated  by  gently  heating  the  object. 

Another  form  of  motion,  which,  however,  does  not  occur  in  living 
cells,  consists  in  an  oscillation  of  minute  granules  within  the  cell.  This 
may  be  due  to  diffusion  currents  or  to  the  Brownian  phenomena.  It 
may  often  be  seen  in  sahvary  corpuscles. 


CELL  DIVISION.  9 

FORMATION  AND  REPRODUCTION  OF  CELLS. 

In  the  past,  two  sorts  of  cell  formation  have  been  recognized,  namely 
the  spontaneous  generation  of  cells,  and  the  origin  of  cells  through  the 
division  of  pre-existing  cells.  According  to  the  theory  of  spontaneous 
generation  it  was  once  thought  that  animals  as  highly  organized  as  intes- 
tinal worms  came  into  existence  from  the  fermentation  of  the  intestinal 
contents.  After  this  had  been  disproved  it  was  still  thought  that  cells 
might  be  formed  directly  from  a  suitable  fluid,  the  cytoblastema.  Some- 
thing of  the  sort  may  have  occurred  when  life  began,  and  it  is  the  expecta- 
tion of  certain  investigators  that  conditions  may  yet  be  produced  which 
shall  lead  to  the  formation  of  organic  bodies  capable  of  growth  and  repro- 
duction. At  present,  however,  only  one  source  of  cells  is  recognized, — 
the  division  of  existing  cells.  "Omnis  cellula  e  cellula."  A  nucleus 
likewise  can  arise  only  by  the  division  of  an  existing  nucleus.  There 
is  no  satisfactory  evidence  that  a  nucleus  may  be  formed  from  non- 
nucleated  protoplasm.  In  cell  division  the  nucleus  divides  first  and  then  the 
protoplasm,  generally  into  two  nearly  equal  parts.  During  the  process 
a  special  grouping  and  transformation  of  the  nuclear  substance  occurs 
in  accordance  v^th  fixed  laws.  The  ordinary  mode  of  cell  division  is 
called  mitosis  or  indirect  division  [karyokinesis]  and  the  characteristic 
groups  of  nuclear  material  are  commonly  known  as  mitotic  figures.  Mitosis 
is  arbitrarily  but  conveniently  divided  into  three  successive  phases,  the 
prophase,  metaphase,  and  anaphase,  in  which  respectively  the  nuclear 
material  prepares  for  division,  divides,  and  returns  to  its  usual  condition. 
(The  final  stages  of  reconstruction  are  often  grouped  as  a  fourth  phase,  the 
telophase.)  In  the  details  of  mitosis  there  are  considerable  variations, 
not  only  in  different  animals  but  also  in  different  kinds  of  cells  in  a  single 
species.  The  account  of  the  process  which  follows  will  apply  only  in  a 
general  way  to  a  particular  case  of  cell  division  which  the  student  may 
be  examining. 

MITOSIS. 

Prophase.  The  centrosome  and  nucleus  approach  one  another  until 
the  centrosome  is  close  to  the  nuclear  membrane  where  it  lies  surrounded 
by  the  clear  zone  of  archoplasm.  The  archoplasm  contributes  to  the 
formation  of  radiating  fibrils  which  extend  from  the  centrosome  in  all 
directions,  and  are  known  collectively  as  the  centrosphere  [astrosphere]. 
The  two  parts  of  the  centrosome  which  had  formed  in  the  "resting  stage" 
by  the  division  of  one,  are  in  the  midst  of  the  centrosphere.     They  move 


lO  HISTOLOGY. 

apart,  the  diplosome  thus  separating  into  two  centrosomes,  and  the  centre- 
sphere  becoming  divided  into  two  spheres,  each  of  which  contains  a  centro- 
some.  Fig.  9. 

The  nucleus  meanwhile  enlarges  and  its  chromatin  stains  much  more 
deeply.  The  branching  portions  of  the  chromatin  network  are  withdrawn, 
so  that  instead  of  a  net,  the  entire  chromatic  material  forms  one  convoluted 
thread,  a  mono  spireme,  as  this  mitotic  figure  is  called.  The  thread  is  at 
first  more  closely  coiled  than  it  is  later.  It  divides  transversely  into  a 
definite  number  of  segments,  called  chromosomes.  These  bodies  may  be 
spherical  or  rod-like,  but  generally  they  are  U-  or  V-shaped.  The  apices 
of  all  the  V's  may  at  first  point  toward  the  centrosome  with  their  free  ends 
directed  away  from  it  as  shown  in  the  diagram,  Fig.  9.  Instead  of  being 
arranged  in  the  orderly  manner  of  the  diagram,  however,  the  chromosomes 

Central  spindle. 
Chromosomes.        Centrosome. 


Fig.  9.— Early  Proph.\se  :  Mono-  Fig.  io.— Later  Prophase  :  Mono- 

SPIREMli.  SPIREME. 

are  so  massed  that  they  can  scarcely  be  counted.  This  is  shown  in  Fig.  15, 
representing  mitoses  in  the  salamander.  In  man  they  are  even  harder  to 
count  and  have  been  estimated  both  as  sixteen  and  twenty-four.  This 
is  of  importance,  since  in  any  one  species  the  number  of  chromosomes 
is  beheved  to  be  constant  for  all  the  cells  except  the  sexual  cells.  Certain 
worms  in  which  the  chromosomes  are  only  two  or  four  in  number  and  hence 
can  be  followed  with  certainty,  have  furnished  the  strongest  evidence 
for  this.  Except  in  the  sexual  cells,  the  number  of  chromosomes  is  always 
even.  Since  it  has  been  found  that  the  same  number  of  chromosomes 
which  entered  into  the  formation  of  the  chromatin  network  of  the  resting 
nucleus,  will  emerge  from  that  net  preceding  mitosis,  the  suggestion  is 
made  that  the  chromosomes  retain  their  individuahty  in  the  quiescent 
nucleus.  They  are  regarded  as  disguised  by  numerous  branches.  In 
the  prophase  of  mitosis  the  chromatin  in  many  cases  does  not  form  a 


MITOSIS.  II 

continuous  thread  but  passes  from  the  network  condition  directly  into  that 
of  a  group  of  chromosomes.  Such  a  group  is,  however,  properly  called 
a  monospireme. 

The  centrosomes,  in  moving  apart  from  one  another,  travel  along 
the  nuclear  membrane  to  points  90°  from  their  original  position.  Thus 
if  before  division  the  centrosome  was  on  one  side  of  the  nucleus,  now  the 
two  centrosomes  into  which  it  has  divided  will  be  found  one  at  either  end 
of  the  nucleus.  Fine  fibrils  extend  between  them  as  they  separate,  con- 
stituting the  central  spindle.  Outside  of  these,  there  are  other  fibrils 
passing  from  the  chromosomes  to  the  centrosomes  (Fig.  10).  These 
fibrils,  which  are  sometimes  derived  from  those  of  the  centrosphere  and 
sometimes  from  the  Hnin  framework  of  the  nucleus,  are  known  as  mantle 
fibrils.  Toward  the  end  of  the  prophase  the  nuclear  membrane  disappears, 
together  with  the  nucleoH. 

Metaphase.      The  V-shaped  chromosomes  become    arranged   about 

Polar  radiation.  Nuclear  spindle. 


Fig.   if.— Early   Metaphase:   Mon-  Fig.  12.— Metaphase  :  Division 

ASTER.  OF  THE   CHROMOSOMES. 

the  equator  of  the  spindle  in  such  a  way  that  their  apices  point  toward  the 
axis  of  the  spindle  and  their  free  ends  radiate  from  it  in  all  directions.  Fig. 
II.  At  either  apex  of  the  spindle  is  the  centrosome  surrounded  by  the 
centrosphere,  the  radiating  fibrils  of  which  are  now  called  polar  radiations. 
If  the  cell  at  this  stage  is  viewed  from  one  of  its  ends  or  poles,  the  chromo- 
somes together  constitute  a  single  star  and  this  mitotic  figure  is  accordingly 
called  the  monaster.  Fig.  15  shows  the  monaster  both  in  side  and  polar 
views. 

In  the  prophase,  before  the  chromosomes  have  formed,  the  convoluted 
thread  of  chromatin  is  sometimes  seen  to  be  split  longitudinally  into 
halves.  During  the  prophase,  therefore,  each  V-shaped  chromosome 
may  consist  of  parallel  portions  which  remain  together  until  the  monaster 
is  complete.     Then,  beginning  at  the  apex  of  the  V,  the  halves  of  each 


12  HISTOLOGY. 

chromosome  are  dra\Mi  apart  as  if  by  means  of  the  outer  spindle,  or  mantle 
fibrils.  In  an  unusual  but  important  form  of  mitosis,  known  as  hetero- 
typical  mitosis,  the  partially  divided  chromosomes  remain  for  some  time 
united  by  their  ends,  in  the  form  of  rings.  How  such  ring-shaped  chromo- 
somes may  occur  is  sho\Mi  in  Fig.  12.  Ordinarily  the  V's  are  completely 
divided,  and  the  separate  halves  travel,  apex  forward,  toward  their  re- 
spective poles.  Two  stellate  groups  are  now  observed  and  this  stage  is 
called  the  dy aster  (Fig.  13).  Stretching  between  these  groups  are  the 
central  fibrils  of  the  spindle,  not  showTi  in  the  drawing.  A  development 
of  granules  in  these  fibrils  along  the  equatorial  plane  may  take  part  in 
forming  a  new  transverse  cell  wall. 

The  metaphase  is  the  stage  of  division  of  the  chromosomes,  and  by 
some  writers  it  is  considered  very  brief,  the  monaster  being  counted  the  last 
of  the  prophase,  and  the  dyaster  being  included  in  the  anaphase. 


Fig.  13. — Latk  Metaphase:  Fig.  14. — Anaphase:  Di- 

Dyaster.  spireme. 

Anaphase.  The  chromosomes  of  either  portion  of  the  dyaster  are  the 
same  in  number  as  those  of  the  nucleus  from  which  they  came.  Each 
group  represents  half  of  the  chromatic  material.  These  new  chromosomes 
unite  with  one  another,  each  group  forming  a  spireme.  The  mitotic 
figure  thus  produced  is  the  dispireme  (Fig.  14).  The  centrosphere  loses 
its  radiations,  becoming  reduced  to  a  zone  of  archoplasm,  and  the  centro- 
some  often  divides  to  form  a  diplosome.  A  nuclear  membrane  forms, 
beginning  at  a  point  opposite  the  centrosomes.  The  nucleoli  reappear 
as  the  chromatin  thread  returns  to  a  network  by  sending  out  branches. 
Thus  two  resting  nuclei  have  formed.  Meanwhile  the  protoplasm  along 
the  equator  constricts,  and  here,  sometimes  aided  by  the  granules  of  the 
central  spindle,  the  new  cell  wall  develops  to  complete  the  process  of  mitosis. 

Summary.  The  stages  described  have  been  successively  the  reticular 
quiescent  stage,  the  monospircme,  monaster,  dyaster,  dispireme,  and  the 
return  to  the  reticular  condition.     These  terms  refer  to  the  arrangement 


MITOSIS. 


13 


of  the  chromatic  material.  The  achromatic  structures  were  successively 
the  centrosome  surrounded  by  archoplasm;  the  diplosome  in  a  centro- 
sphere ;  two  centrosomes  connected  by  a  spindle  and  each  surrounded  by 
polar  radiations ;  the  division  of  this  amphiasUr,  as  it  is  called,  into  two  cen- 
trospheres  each  with  its  centrosome ;  and,  finally,  the  reduction  of  the  cen- 
trosphere  to  archoplasm.  Each  new  cell  ordinarily  receives  half  of  the 
protoplasm,  spindle,  centrosome  and  chromatic  material  of  its  parent, 
and  becomes  a  cell  of  the  same  sort. 

The  process  of  mitosis  requires  probably  about  half  an  hour,  but 
the  time  is  variable  and  it  may  last  several  hours.  In  the  blood  cells  of 
amphibia  it  is  said  to  take  two  hours  and  a  half.     Mitoses  will  be  found 

Close  monospireme  Loose  monospireme 

(viewed  from  (viewed  from  above — 

the  side).  i.  e.,  from  the  pole).        Monaster  (viewed  from  the  side). 
Polar  side. 


^?^Si. 


^Wfr-, 


Polar 
radiation. 


Spindle. 


Monaster  (viewed 
from  above). 


Beeinnins 


Completed, 


Division  of  the  protoplasm  (Dispiremes). 


Fig.  15. — Mitotic  Figures  from  the  Epithelium  of  the  Oral  Cavity  of  Triton 

Alpestris.    X  560. 

in  all  well  preserved,  rapidly  developing  tissues.  They  are  abundant 
in  embryos;  and  if  numerous  in  tumors  they  furnish  evidence  of  rapid 
growth  and  maHgnancy.  After  death,  if  the  tissues  are  not  hardened  by 
cold  or  reagents,  it  is  thought  that  mitoses  may  go  on  to  completion,  as 
they  are  absent  from  specimens  which  are  not  properly  preserved. 

Varieties  of  mitosis.  In  connection  mth  the  formation  of  sexual 
cells  (the  ova  and  spermatozoa)  there  occur  two  successive  mitotic  divisions 
of  a  unique  sort.  A  cell  which  had  itself  been  formed  by  ordinary  mitosis, 
in  preparing  for  division  converts  its  chromatic  material  into  one  half  of 
the  usual  number  of  chromosomes.  It  divides  into  two  cells,  each  with  the 
reduced  number,  and  these  divide  once  more  in  the  same  way.     Thus 


14 


HISTOLOGY. 


four  cells,  each  having  one  half  the  usual  number  of  chromosomes,  arise 
from  the  one  which  first  presented  this  peculiarity.  With  some  modifi- 
cations but  without  further  division  they  may  become  the  mature  sexual 
cells.  The  process  of  their  formation  is  called  maturation,  and  the  two 
pecuHar  and  final  mitoses  through  which  every  mature  sex  cell  has  passed 
are  called  reduction  divisions.  In  the  process  of  fertilization  two  mature 
sexual  cells,  a  spermatozoon  and  ovum  respectively,  fuse,  and  the  normal 
number  of  chromosomes  is  restored.  Thus  each  parent  contributes  an 
equal  number  of  chromosomes  to  the  fertilized  ovum  and  these  have  been 
considered  bearers  of  hereditary  qualities.  The  reduction  divisions  will 
be  further  considered  under  Testis  and  Ovary, 

An  unusual  form  of  mitosis  is  that  in  which  the  centrosome  divides 
into  more  than  two  parts  and  the  cell  correspondingly  divides  at  once  into 
several.  These  pluri-  or  multi-polar  mitoses  are  said  to  occur  normally 
in  parts  of  certain  higher  plants;    they  have  been  induced  by  injecting 


Fig.   i6. — Mitoses   in  Human  Cancer  Cells.     (From  Wilson,  after  Galeotti.) 
a,  Asymmetrical  mitosis  with  unequal  distribution  of  chromatin;  b,  tripolar  mitosis;  Ci  quadripolar  mitosis. 

drugs  into  the  skin  of  salamanders;  and  are  sometimes  found  in  human 
cancer  cells  and  in  the  rapidly  growing  coimective  tissue  of  scars.  They 
may  lead  to  an  unequal  distribution  of  the  chromatic  material  in  the  cells 
which  they  produce. 

For  further  information  regarding  mitosis,  and  for  definition  of  the 
many  terms  frequently  employed  but  not  mentioned  in  this  account,  the 
student  is  referred  to  Prof.  E.  B.  Wilson's  book  entitled  "The  Cell." 

AMITOSIS. 

Amitosis  or  direct  cell  division  takes  place  without  spindle  formation 
or  the  rearrangement  of  nuclear  material.  The  nucleolus,  nucleus,  and 
cell  body  successively  divide  by  fission,  or  by  elongation  and  constriction, 
into  two  parts.  The  role  of  the  centrosome  has  not  been  determined. 
This  form  of  divison  is  rare  and  its  significance  unknown.  The  suggestion 
that  it  is  more  primitive  than  mitosis  lacks  support.  Generally  it  is 
regarded  as  a  sign  of  cell  degeneration,  since  it  occurs  in  old  cells — leuco- 


CYTOMORPHOSIS. 


15 


cytes  and  the  superficial  cells  of  the  bladder— the  cell  bodies  of  which  often 
fail  to  divide  following  the  division  of  their  nuclei.  Thus  cells  with  two 
or  more  nuclei  may  be  produced  by  amitosis.     It  occurs  in  wounded 


Beginning  Completed  Beginning  Completed 

Division  of  the  nucleolus.  Division  of  the  nucleus. 

Fig.  17. — Amitosis  IN  Epithelial  Cells  from  the  Bladder  of  a  Mouse.    X  560. 

tissues  where  it  has  been  interpreted  both  as  a  result  of  injury  and  as 
evidence  of  activity  toward  repair.  In  the  egg  tubes  of  certain  insects 
amitosis  is  a  common  and  normal  process. 

CYTOMORPHOSIS. 

Cytomorphosis  is  a  comprehensive  term  for  the  structural  modifications 
which  cells  or  successive  generations  of  cells  may  undergo  from  their 
origin  to  their  final  destruction.  It  impHes  that  the  life  of  a  cell  is  hmited, 
and  that  during  its  life  it  may  change  in  structure  by  becoming  differentiated, 
or  adapted  to  the  performance  of  special  functions,  and  that  finally  it  will 
pass  through  regressive  changes  to  its  death.  Successive  generations  of 
cells  may  represent  stages  along  a  certain  line  of  differentiation.  The 
cells  resulting  from  mitotic  division  begin  their  specialization  where  the 
parent  cell  left  off,  and  the  phenomena  of  regression  are  then  reserved 
for  the  final  generations  in  the  series.  Four  successive  stages  of  cytomor- 
phosis have  been  recognized:  First,  the  undifferentiated  stage;  second, 
that  of  progressive  differentiation;  third,  the  stage  of  regression;  fourth, 
the  removal  of  dead  material.     These  may  be  considered  in  turn. 

Undifferentiated  cells,  as  can  be  seen  in  sections  of  young  embryos, 
are  characterized  by  large  nuclei  and  relatively  little  protoplasm.  They 
have  great  power  for  undergoing  division.  The  subsequent  increase 
of  cytoplasm  which  makes  functional  differentiation  possible,  retards  the 
rate  of  mitosis.  In  the  adult,  relatively  undifferentiated  ceUs  are  found 
in  many  situations,  as,  for  example,  in  the  deep  layer  of  the  epidermis. 
These  cells  are  a  source  of  supply  to  replace  the  outer  cells  as  they  differ- 


i6 


HISTOLOGY. 


entiate,  die,  and  are  cast  off.  Since  they  can  produce  only  epidermal  cells, 
they  are  themselves  partly  differentiated.  The  fertilized  o\^m  which  can 
produce  all  kinds  of  cells  must  be  regarded,  in  spite  of  its  size  and  great 
mass  of  yolk-laden  cytoplasm,  as  the  least  differentiated. 

The  progressive  specialization  of  cells  concerns  chiefly  their  proto- 
plasm, yet  in  the  case  of  the  muscle  fibers  of  the  salamander  it  is  accom- 
panied also  by  marked  nuclear  changes.  Typical  muscle  nuclei  from 
Nectunis  embryos  7  mm.  and  26  mm.  long,  respectively,  are  sho^^'n  in  Fig. 
18.     The  significance  of  the  differences  between  them  is  not  known,  as 

they  have  been  but  recently  de- 
tected. The  cytoplasm  of  muscle 
cells  differentiates  its  contractile 
function  beyond  all  others,  and 
becomes  filled  with  contractile 
fibrils.  Many  kinds  of  cells  are 
specially  modified  for  producing 
secretions  which  may  either  be 
discharged,  as  from  gland  cells, 
or  in  a  somewhat  solid  state 
may  remain  in  contact  with  the 
cell,  thus  forming  certain  of  the 
intercellular  substances.  Small 
amounts  of  structureless  intercel- 
lular substance,  such  as  is  some- 
times found  between  epithelial 
cells,  are  called  cement  substance, 
even  though  it  may  be  fluid. 
Between  connective  tissue  cells 
the  intercellular  substances  are 
formed  in  such  quantity  that  they 
far  exceed  the  bulk  of  the  cells 
which  produced  them.  These 
ground  substances  may  be  homogeneous,  or  permeated  with  fibrils  and 
granules,  formed  either  by  the  exoplasm  or  by  the  transformation  of 
the  intercellular  substance.  The  remnant  of  ground  substance  between 
the  fibrils  is  another  so-called  cement  substance.  In  cartilage  and  bone, 
the  cells  appear  scattered  through  the  ground  substance  which  by  their 
differentiation  they  have  produced. 

Regression  or  degeneration  is  the  manifestation  of  approaching  death. 
Normally  it  is  not  seen  in  nerve  cells  and  probably  not  in  the  voluntary 
muscle  cells.     Subtle  and  unrecognized  changes  may  occur  in  them  in 


A  B 

Fig.  18.— Nuclei  of  Striated  Musclk  Fibf.rs  from 

Young  Salamanders  (Necturls).     (Eyclcshy- 

mer.) 
A,  From  a  7  mm.  embryo ;  B,  from  one  of  26  mm.;  ch., 

chromaliii  knot;  g.  8.,  ground  subslaiict ;   I,  liiiiii 

fibril;  n.,  nucleolus;  n.m.,  nuclear  membrane. 


CYTOMORPHOSIS.  17 

old  age,  but  they  remain  active  throughout  Kfe;  if  destroyed,  they  can 
never  be  replaced.  In  many  glands,  in  the  blood  and  in  the  skin,  however, 
the  cells  are  constantly  dying  and  new  ones  are  being  differentiated.  In 
a  few  organs  the  cells  perish,  but  no  new  ones  form,  so  that  the  organ  to 
which  they  belong  atrophies.  Thus  the  mesonephros  (Wolffian  body) 
largely  disappears  during  fetal  life;  the  thymus  becomes  vestigial  in  the 
adult;  and  the  ovary  in  later  years  loses  its  chief  function  through  the 
degeneration  of  its  cells. 

The  optical  effects  of  regression  cannot  at  present  be  properly  classi- 
fied. In  a  characteristic  form,  known  as  "cloudy  swelling,"  the  cell  en- 
larges, becoming  pale  and  opaque.  In  another  form  the  cell  shrinks  and 
stains  deeply,  becoming  either  irregularly  granular  or  homogeneous  and 
hyaline.  The  nucleus  may  disappear  as  if  in  solution  (karyolysis,  chromato- 
lysis),  or  it  may  fragment  and  be  scattered  through  the  protoplasm  (karyo- 
rhexis).  If  the  process  of  degeneration  is  slow,  the  cell  may  divide  by 
amitosis.  It  may  be  able  to  receive  nutriment  which  it  cannot  assimilate, 
and  thus  its  protoplasm  may  be  infiltrated  with  fat  and  appear  vacuolated. 
It  may  form  abnormal  intercellular  substances,  for  example,  amyloid,  or 
the  existing  intercellular  substances  may  become  changed  to  mucoid 
masses  or  have  lime  salts  deposited  in  them.  In  short,  together  with 
optical  changes  in  the  cell  substance  there  is  often  an  impairment  or 
perversion  of  function. 

The  removal  of  dead  cells  is  accomplished  in  several  ways.  Those 
near  the  external  or  internal  surfaces  of  the  body  are  usually  shed  or  des- 
quamated, and  such  cells  may  be  found  in  the  saliva  and  urine.  Those 
which  are  within  the  body  may  be  dissolved  by  chemical  action  or  devoured 
by  phagocytes. 

Every  specimen  of  human  tissue  exhibits  some  phase  of  cytomorphosis. 
In  some  sections  a  series  of  cells  may  be  observed  from  those  but  slightly 
differentiated,  to  the  dead  in  process  of  removal.  Because  of  the  similarity 
and  possible  identity  of  this  normal,  "physiological"  regression,  with  that 
found  in  diseased  tissues,  such  specimens  should  be  studied  with  particular 
care. 


HISTOLOGY. 


II.  GENERAL  HISTOLOGY. 
HISTOGENESIS. 

Segmentation  and  the  Formation  of  the  Germ  Layers. 

The  body  is  composed  of  groups  of  similarly  differentiated  cells, 
similar  therefore  in  form  and  function.  Such  groups  are  knov^Ti  as  tissues. 
Histology  (Greek,  ''(^roc,  "a  textile  fabric")  is  the  science  of  tissues, 
and  histogenesis  deals  with  their  origin.  There  are  as  many  tissues  in  the 
body  as  there  are  "sorts  of  substance";  thus  the  liver  consists  essentially 
of  hepatic  tissue,  and  the  bones  of  osseous  tissue.  All  of  these,  however,  are 
modifications  of  a  small  number  of  fundamental  tissues,  the  histogenesis 
of  which  may  now  be  considered. 

It  has  already  been  noted  that  a  new  human  individual  begins  existence 
as  a  single  cell,  the  fertihzed  o\aim,  formed  by  the  fusion  of  two  mature 
sexual  cells,  the  spermatozoon  and  o\aim  respectively.  The  fertilized 
ovum  then  divides  by  mitosis  into  a  pair  of  cells.  Fig.  19,  A;  these  again 
divide  making  a  group  of  four.  Fig.  19,  B;  by  repeated  mitosis  a  mulberry- 
like mass  of  cells  results,  called  the  morula,  Fig.  19,  C.  Development  to 
this  point  is  known  as  the  segmentation  of  the  ovum. 

A  section  through  the  morula  is  shown  in  D.  An  outer  layer  of  cells 
surrounds  the  inner  cell  mass.  Soon  a  cup-shaped  cleft,  crescentic  in 
vertical  section,  forms  between  the  outer  and  inner  cells  as  shown  in  E,  and 
this  enlarges  until  the  entire  structure  becomes  a  single-layered,  thin- walled 
vesicle,  within  and  attached  to  one  pole  of  which  is  the  inner  cell  mass. 
This  mass  gradually  spreads  beneath  the  outer  layer  until  it  forms  a 
complete  lining  for  the  vesicle,  which  becomes  consequently  two  layered, 
Fig.  19,  G.  The  inner  layer  is  called  entoderm,  and  the  outer  layer, 
ectoderm.*  The  entire  embryonic  structure  at  this  stage  is  called  a 
blastodermic  vesicle. 

On  the  upper  surface  of  the  vesicle  the  future  -axis  of  the  embryo  is 
indicated  by  a  thickened  streak  called  the  primitive  streak.  In  front  of 
this  there  is  a  groove  in  the  ectoderm,  also  in  the  axial  Hne  of  the  future 
body.     It  is  named  the  medullary  groove,  and  just  beneath  it  is  a  rod 


*  The  ectoderm  is  in  part  derived  from  the  superficial  cells  of  the  inner  cell  mass,  and 
in  part  from  the  primary  outer  layer  of  the  vesicle.  The  former  portion  is  to  cover  the  body 
of  ^ the  embryo,  and  the  latter  [named  trophoblast]  covers  the  fetal  membranes.  These 
membranes  are  to  be  described  in  a  later  chapter.  They  are  omitted  in  the  diagram? 
of  Fig.  xg. 


GERil   LAYERS. 


19 


of  entodermal  cells  called  the  notochord.     These  may  be  seen  in  cross 
section  in  Fig.  19,  G  and  H.     In  G,  on  either  side  of  the  medullary  groove 


Neph. 


Fig.  19.— Diagrams  showixg  the  Development  of  the  Germ  Layers.    (A  to  F,  after  van  Beneden's 

figures  of  the  rabbit.) 

A,  Two-celled  stage ;  B,  four-celled  stage ;  and  C,  morula  stage  of  the  segmenting  ovum,  all  being  surface 
views.  D  to  I  represent  sections  described  in  the  text.  The  inner  cell  mass  and  entoderm  are  heavily 
.shaded;  \.\\&  outer  layer  3.\\^  ectoder7n-ss^\\%\i\.\  and  \}ix&7nesoderm\s  represented  by  dashes.  Coe., 
coelom  or  body  cavity.     Int.,  intestinal  cavity.     Neph.,  nephrotome.     Seg.,  mesodermic  segment. 

and  notochord  a  third  layer  of  cells  appears  between  the  ectoderm  and 
entoderm,  and  it  gradually  encircles  the  vesicle  as  did  the  entoderm.  It 
is  the  mesoderm,  which  has  an  obscure  origin  near  the  primitive  streak. 


20  HISTOLOGY. 

As  it  spreads  out  around  the  vesicle  it  divides  into  two  layers,  one  of  which 
is  closely  applied  to  the  ectoderm  (the  somatic  layer)  and  the  other  to  the 
entoderm  (the  splanchnic  layer) .  Between  them  is  the  body  cavity  or  coelom, 
which  in  the  adult  is  subdivided  into  the  peritoneal,  pleural,  and  pericardial 
cavities.  The  ectoderm  and  the  somatic  mesoderm  constitute  the  somato- 
pleure,  or  body  wall;  the  entoderm  and  splanchnic  mesoderm  form  the 
intestinal  wall,  or  splanchnopleure.  The  coelom  has  appeared  in  Fig.  19, 
H,  and  in  I  it  has  attained  a  full  development.  On  the  ventral  side  of  the 
intestine  it  crosses  the  median  line.  Dorsally  the  medullary  groove, 
which  has  now  become  the  medullary  tube  by  the  fusion  of  its  upper 
margins,  separates  the  coelom  into  right  and  left  portions.  Fig.  19,  I, 
may  be  regarded  as  showing  the  fundamental  relations  to  be  observed  in 
the  cross  section  of  an  adult,  made  through  the  abdominal  cavity. 

Reviewing  the  preceding  paragraphs  it  is  seen  that  the  fertihzed 
ovum  through  segmentation  forms  a  morula,  and  later  a  blastodermic 
vesicle  composed  of  three  germ  layers,  the  ectoderm  or  outer,  the  mesoderm 
or  middle,  and  the  entoderm  or  inner.  For  studying  the  transformation 
of  these  layers  into  the  organs  and  tissues  of  the  adult,  chick  embryos 
are  more  available  than  those  of  mammals.  The  structure  of  a  chick 
embryo  of  about  thirty  hours'  incubation  may  therefore  be  briefly  reviewed. 
Fig.  20,  A,  represents  a  dorsal  view  of  such  an  embryo,  various  portions 
of  which  have  been  removed,  and  Fig.  20,  B,  is  a  median  sagittal  section  of 
a  similar  embryo.  On  the  dorsal  side  the  ectoderm  forms  a  continuous 
layer  covering  the  embryo,  and  it  becomes  a  part  of  the  skin, — the  epi- 
dermis and  its  appendages.  In  the  figure  (A)  it  has  been  cut  away  except 
a  portion  folded  in  under  the  head  and  the  part  surrounding  the  rhomboidal 
sinus,  rh.s.  Besides  the  epidermis  the  ectoderm  forms  the  medullary 
groove,  the  edges  of  which  unite  to  form  the  tube  beginning  near  the 
head.  The  union  of  these  edges  proceeds  in  both  directions.  The 
anterior  neuropore  is  the  last  portion  to  close  anteriorly  (there  are  two 
small  anterior  openings  in  B),  and  the  rhomboidal  sinus  is  the  expanded 
open  part  behind.  Later  these  openings  are  closed  over  and  the  medul- 
lary tube  becomes  detached  from  the  epidermis.  Its  anterior  part 
enlarges  to  form  the  brain  and  the  two  optic  vesicles  (op.  v.),  each  of  which 
becomes  the  retina  of  an  eye.     Its  posterior  part  forms  the  spinal  cord. 

The  entoderm  in  dorsal  view  is  the  deepest  layer,  exposed  by  removing 
the  ectoderm  and  mesoderm.  Under  the  head  it  forms  a  broad  finger- 
like pocket,  the  pharynx  (ph.).  Its  relations  are  seen  in  the  median  section. 
Later  its  anterior  end  fuses  with  an  inpocketing  of  the  adjacent  ectoderm 
to  form  the  oral  plate.  When  this  plate  becomes  thin  and  ruptures,  the 
pharynx  opens  to  the  exterior  at  the  mouth.     Posteriorly  the  entoderm 


GERM   LAYERS. 


21 


envelops  the  yolk  mass  which  may  be  regarded  as  occupying  a  distended 
intestine.  The  entoderm  forms  the  hning  of  the  pharynx  and  intestine, 
together  with  their  appendages  which  include  the  lungs,  Hver,  pancreas, 
and  bladder.  These  develop  later.  The  intestine  acquires  its  anal 
opening  by  the  rupture  of  an  anal  plate,  formed,  like  the  oral  plate,  by  the 
meeting  of  entoderm  and  ectoderm.     The  entoderm  also  gives  rise  to  the 


A  B 

Fig.  20. — Diagrams  Based  upon  Reconstructions  of  \  Chick  of  30  Hours. 
A,  Dorsal  view.  B,  Median  sagittal  section  but  with  the  entire  heart,  ant.  n.,  Anterior  neuropore; 
ao.,  aorta;  ect.,  ectoderm;  ent.,  entoderm;  Ht.,  heart;  med.gr.,  med.  tube,  medullary  groove  and 
tube;  mes.  seg.,  mesodermic  segment;  nch.,  notochord  ;  neph.,  nephrotome  ;  op.  v.,  optic  vesicle; 
p.  cav.,  pericardial  cavity;  ph.,  pharynx;  pp.  St.,  primitive  streak;  rh.  s.,  rhomboidal  sinus;  som. 
mes.,  spl.  mes.,  somatic  and  splanchnic  mesoderm  ;  v.  v.,  vitelline  \ein. 


notochord,  a  supporting  rod  of  cells  extending  from  the  anterior  end  of 
the  primitive  streak,  along  the  axial  line  to  the  head  (B,  nch.).  It  is  the 
only  skeletal  element  in  some  animals.  In  fishes  it  is  retained  as  agelatinous 
cord  running  through  the  bodies  of  the  vertebrae  which  have  formed  about 
it,  and  expanding  in  the  intervertebral  spaces.    In  man,  if  it  remains  at  all 


22  HISTOLOGY. 

it  is  vestigial  in  the  adult.    It  sometimes  develops  abnormally,  forming  a 
peculiar  tumor. 

The  mesoderm  has  been  described  as  forming  splanchnic  and  somatic 
layers  which  unite  with  one  another  toward  the  median  line.  WTiere  the 
layers  come  together  they  are  greatly  thickened,  and  the  thickened  portion, 
by  a  series  of  transverse  constrictions,  becomes  cut  into  block-like  masses 
called  mesodermic  segments  (protovertebrae).  They  are  paired  structures 
bordering  upon  the  medullary  tube  and  increasing  in  number  by  the  forma- 
tion of  new  segments,  chiefly  posteriorly.  A  portion  of  them  is  seen  on 
the  right  of  Fig.  20,  A;  the  rest  have  been  removed.  There  is  a  longi- 
tudinal depression  separating  the  segments  from  the  splanchnic  and   so- 


som 


[Fig.  21. — Transverse  Section  of  a  2.5  mm.  Hc.man  E.mbryo.     (After  von  Lenhossek.) 
(Compare  this  section  with  the  upper  part  of  the  diagram,  Fig.  19,  I.) 
Ac,  aorta  ;  coe.,  coelom  ;  ecL, ectoderm  ;  ent.,  entoderm  ;  int.,  intestinal  cavity  ;  med.  t.,  medullary  tube  ; 
nch.,  notochord ;    neph.,  nephrotome;    seg.,  mesodermic  segment;    som.,  somatic   mesoderm;    spl.. 
splanchnic  mesoderm. 

matic  layers,  and  the  part  of  mesoderm  which  crosses  the  depression  is 
called  the  intermediate  cell  mass,  or  7iephrotome.  The  coelom  at  first 
extends  through  the  nephrotome  into  the  segments,  as  shown  in  the  cross 
section.  Fig.  19, 1.  Later  the  segments  and  nephrotome  become  separated 
from  the  lateral  layers  and  from  each  other,  and  lose  their  cavities.  This 
has  occurred  in  the  nephrotome  of  Fig.  21.  From  the  cells  of  the  segments 
the  voluntary,  striated  muscles  are  derived,  and  from  the  nephrotomes 
come  the  lining  layer  of  the  genital  and  urinary  ducts  and  kidneys.  From 
all  parts  of  the  mesoderm  certain  cells  become  detached,  and  then  unite 
with  one  another  by  branching  protoplasmic  processes.  Thus  they  form 
a  network,  in  the  meshes  of  which  is  a  clear  intercellular  fluid.     Such 


GERM   LAYERS. 


23 


M.T. 


tissue  is  called  mesenchyma  (Fig.  22).  It  fills  the  intervals  between  the 
layers  already  described  and  surrounds  the  notochord  and  medullary 
tube.  Mesenchymal  cells,  however,  do  not  enter  the  coelom.  In  the 
chick  embryo  of  Fig.  20,  A,  the  greatest  accumulation  of  mesenchyma 
would  be  found  between  the  ectoderm  covering  the  head  and  the  medullary 
tube.  Both  the  cells  and  the  intercellular  substance  of  mesenchyma 
undergo  transformations;  the  latter  mav  become  a  more  or  less  solid 
matrix.  Thus  mesenchyma  produces  cartilage  and  bone,  tendon,  fascia, 
and  the  loose  connective 
tissue  through  which  the 
vessels  and  nerves  extend, 
together  with  smooth 
muscle  fibers  and  fat. 

In  the  splanchno- 
pleure,  between  the  meso- 
dermal and  entodermal 
layers,  a  network  of  blood 
vessels,  lined  with  very 
flat  ceUs,  appears  early 
in  embryonic  Hfe  (Fig. 
23).  Its  first  indication 
is  the  formation  of  irregu- 
lar dark  patches  of  cells, 
caUed  hlood  islands, -which. 
surround  the  embryo  as 
a  mottled  layer.  The 
islands  consist  of  cells 
which  form  the  blood  cor- 
puscles, and  perhaps  also 
the  lining  of  the  blood 
vessels  which  surround 
them.  So  distinct  is  this 
vascular  layer  that  it  has 
been  called  the  angioblast, 

and  regarded  as  a  separate  germ  layer.  Usually  it  is  considered  to  be  de- 
rived from  the  mesenchyma.  After  the  angioblast  has  once  been  developed 
it  sends  prolongations  into  the  embryo  to  form  the  bloodvessels.  The  latter 
thereafter  never  arise  from  mesenchymal  spaces,  but  always  as  sprouts 
from  the  pre-existing  vessels,  growing  through  mesenchyma  like  roots 
through  the  soil.  In  single  sections  the  lining  of  the  vessels  may  appear 
inseparable  from  the  cells  around  them,  as  in  Fig.  22,  but  by  following 


B.v: 


Fig.  22. — Sectiok  from  the  Head  of  a  Rabbit  Embryo  of  io^ 
Days,  4.4  mm.,  to  Show  Mesenchyma. 

Epi.  and  M.  T.,  Ectodermal  epithelium  of  the  epidermis  and  medul- 
lary tube,  respectively.  N.,  nucleus  ;  P.,  protoplasm  ;  and  I.  S., 
intercellular  substance  of  a  mesenchymal  cell.  Two  of  these 
cells  show  mitotic  figures.  B.  V.,  Blood  vessel,  lined  by  endo- 
thelium. One  of  the  blood  vessels  contains  an  embryonic  red 
blood  corpuscle. 


24 


HISTOLOGY. 


the  vessels  from  section  to  section  they  will  be  found  to  be  branches. 
The  red  blood  corpuscles  of  the  adult  are  thought  to  be  descendants  of 
those  which  form  the  blood  islands.  They  multiply  in  places  to  which 
they  have  been  carried  by  the  circulating  blood,  for  example  in  the  liver 
in  later  embryonic  life,  and  in  the  red  bone  marrow  of  the  adult.  The 
white  corpuscles  may  be  derived  from  the  same  parent  form  as  the  red, 
or  they  may  have  several  origins.  The  corpuscles  pass  out  between  the 
cells  of  the  vessel  walls  into  the  mesenchyma,  where  they  wander  about. 
Whether  some  of  them  are  formed  by  the  transformation  of  mesenchymal 
cells  is  still  discussed.  Their  earliest  origin  hke  that  of  the  vessel  walls 
is  obscure. 

The  vascular  system  in  the  chick  embryo  (Fig.  20)  consists  of  the 
network  in  the  splanchnopleure  just  over  the  yolk,  from  which  nutriment 
is  received  by  .the  blood.     This  is  conveyed  by  the  vitelline  veins,  one  on 


Ent 


Fig.  23. — Wall  of  the  Yolk  Sac  (Intestine)  from  a  Chick  of  the  Second  Day  of 

Incubation.    (Minot.) 

Mes.,  Splanchnic  mesoderm  ;  Ent.,  entoderm,  four  distinct  cells  of  which  are  shown  at  c  ;  V.  V,,  blood 

vessels  containing  a  few  young  blood  cells. 

either  side,  to  the  heart,  a  single  median  vessel  under  the  pharynx  made 
by  the  junction  of  the  veins  (Fig.  20,  B).  The  heart  divides  into  two 
aortae  which  pass  around  the  anterior  end  of  the  pharynx  to  its  dorsal  side 
and  then  extend  through  the  body  posteriorly,  lying  under  the  segments. 
Their  branches  pass  off  laterally  to  the  vitelline  network,  thus  completing 
the  circulation.  All  future  vessels  in  the  body  are  branches  of  this  simple 
system. 

The  Fundamental  Tissues. 

It  has  been  said  that  there  are  two  fundamental  tissues,  epithelium 
and  mesenchyma.  Epithelium  is  a  layer  of  cells  covering  an  external  or 
an  internal  surface  of  the  body,  having  one  side  free  and  the  other  resting 
on  underlying  tissue.  The  epidermis,  and  the  linings  of  the  intestinal 
tract,  of  the  blood  vessels,  of  the  peritoneal  cavity  and  of  the  joint  cavities 


THE    FUXDAMEXTAL    TISSUES.  25 

are  all  examples  of  epithelia.  The  epidermis  is  ectoderm;  the  lining  of 
the  intestine  is  entoderm;  that  of  the  blood  vessels,  called  endothelium,  is 
from  the  angioblast;  the  peritoneal  epithehum  (mesothelium)  is  part  of 
the  splanchnic  and  somatic  layers  of  mesoderm;  and  the  joint  ca\dties  are 
lined  by  flattened  mesenchymal  cells,  the  cavity  being,  as  it  were,  a  large 
intercellular  space.     Thus  epitheha  are  derived  from  all  the  germ  layers. 

Mesenchyma  is  a  non-epithelial  portion  of  the  mesoderm,  which 
has  just  been  described  as  consisting  of  branched  cells,  the  protoplasmic 
processes  of  which  form  a  continuous  network.  In  its  meshes  is  a  clear 
intercellular  fluid.  ^Mesenchyma  is  essentially  a  tissue  of  the  embr}'o. 
In  the  adult  it  is  represented  by  connective  tissue,  bone,  and  other  deriva- 
tives which  preserve  certain  of  the  characteristics  of  mesenchyma. 

Three  other  forms  of  tissue  depart  so  far  from  the  epithehal  and 
mesenchATTial  t}'pes  that  they  are  naturally  placed  by  themselves.  These 
are  muscle,  nerv^e,  and  vascular  tissue.  Muscle  tissue  exists  in  three  forms, 
of  which  the  smooth  and  cardiac  varieties  are  derived  from  mesench}Taa, 
and  the  striated  Cvokmtaryj  muscles  from  the  mesodermic  segments.  The 
epithehal  character  of  the  latter  is  lost.  Nerve  tissue  is  ectodermal,  con- 
sisting of  an  epithehal  tube  which  later  becomes  essentially  non-epithelial, 
and  of  detached  masses  of  cells  which  send  processes  to  all  parts  of  the 
body,  forming  the  nerves.  These  are  never  epithelial.  Vascular  tissue 
includes  the  blood  and  the  l}Tnph,  which  are  of  obscure  origin,  perhaps 
mesench}Tiial;  also  the  endothehum  which  lines  the  vessels,  provided  that 
the  blood  and  the  endothehum  have  a  common  origin.  It  will  be  con- 
venient to  describe  the  entire  blood  vessels  and  hmphatic  vessels  in  connec- 
tion mth  their  contents. 

In  the  following  pages  the  several  tissues  wiU  be  considered  in  the 
order  above  outhned.  In  connection  with  them,  certain  organs  may  be 
examined.  An  organ  is  a  more  or  less  independent  portion  of  the  body, 
ha\dng  its  own  blood,  lymphatic  and  nerv'e  supply,  and  connective  tissue 
framework,  together  Vvdth  its  characteristic  essential  cells.  Thus  an  organ 
should  consist  of  several  tissues.  The  pancreas  or  lungs  are  obviously 
organs.  An  individual  muscle  or  a  particular  bone  has  a  connective 
tissue  framework  or  covering,  blood  vessels  and  nen-es,  besides  its  essential 
substance.  Thus  it  is  an  organ.  Even  a  blood  vessel  of  ordinary  size 
comes  within  the  definition.  The  organs  which  are  of  wide  occurrence 
Hke  the  bones,  muscles,  tendons,  nerves  and  vessels,  may  be  described 
with  their  essential  tissues.  The  more  complex  organs  are  reser\-ed  for 
the  later  section  entitled  "Special  Histology." 

Before  presenting  in  summary  form  the  derivatives  of  the  germ  layers 
it  should  be  noted  that  the  ectoderm  becomes  continuous  with  the  ento- 


26  HISTOLOGY. 

derm  at  the  mouth,  anus,  and  urogenital  opening.  The  hne  of  separation 
is  not  that  of  transition  from  skin  to  mucous  membrane,  but  is  indicated 
by  the  transient  membranes  (the  oral  and  anal  plates)  found  in  young 
embryos.     Nothing  in  the  adult  remains  to  show  where  the  layers  join. 

ORIGIN  OF  THE  TISSUES  FROM  THE  GERM  LAYERS. 
The  ectoderm  produces : 

1.  Epithelium  of  the  following  organs: — the  skin,  including  its  glands, 
hair  and  nails ;  the  cornea  and  the  lens ;  the  external  and  internal  ear ;  the  nasal 
and  oral  cavities,  including  the  salivary  glands,  the  enamel  of  the  teeth  and  an- 
terior lobe  of  the  hypophysis;  the  anus;  the  cavernous  and  membranous  parts  of 
the  male  urethra;  together  with  that  epithelium  of  the  chorion  which  is  toward 
the  uterus  and  of  the  amnion  which  is  toward  the  fetus. 

2.  Nerve  tissue  forming  the  entire  nervous  system,  central,  peripheral 
and  sympathetic. 

3.  Muscle  tissue,  rarely,  as  of  the  sweat  glands,  and  perhaps  also  some 
muscle  fibers  of  the  iris. 

The  mesoderm  produces : 

1.  Epithelium  of  the  following  structures: — the  urogenital  organs  except 
most  of  the  bladder  and  the  urethra ;  the  pericardium,  pleurae,  and  peritonaeum 
and  the  continuation  of  this  layer  over  the  contiguous  surfaces  of  amnion  and 
chorion;    the  blood  and  lymphatic  vessels;   and  the  joint  cavities  and  bursae. 

2.  Muscle  tissue,  striated  (voluntary),  cardiac,  and  smooth  (involuntary). 

3.  Mesenchyma,  an  embryonic  tissue,  which  forms  in  the  adult,  connective 
and  adipose  tissue,  bone  (including  the  teeth  except  their  enamel),  cartilage, 
tendon,  and  various  special  cells. 

4.  Vascular  tissue,  the  cells  of  the  blood  and  lymph,  consequently  the 
essential  elements  of  the  lymph  glands,  red  bone  marrow  and  spleen. 

The  entoderm  produces: 

I .  Epithelium  of  the  following  organs : — the  pharynx,  including  the  auditory 
tube  and  middle  ear,  thyreoid  and  thymus  glands;  the  respiratory  tract,  including 
larynx,  trachea,  and  lungs;  the  digestive  tract,  including  the  oesophagus,  stomach, 
small  and  large  intestine,  rectum,  liver,  pancreas,  and  the  fetal  yolk  sac;  and 
part  of  the  urinary  organs,  namely  most  of  the  bladder,  the  female  urethra,  and 
prostatic  part  of  the  male  urethra. 

2    NoTOCHORDAL  TISSUE,  which  disappears  ( ?)  in  the  adult. 

EPITHELIA. 
Epithelium  has  already  been  defined  as  a  layer  of  cells  covering  an 
external  or  an  internal  surface  of  the  body,  having  one  side  therefore  free, 
and  the  other  resting  on  underlying  tissue.  Epithelia  differ  from  one 
another  in  embryonic  origin,  in  the  shape  of  their  cells,  in  the  number 
of  layers  of  cells  of  which  they  are  composed,  and  in  the  differentiation 
of  these  cells.  All  of  these  features  should  be  recorded  in  any  complete 
description  of  an  epithelium,  and,  except  the  origin,  something  of  each 
is  to  be  observed  in  a  single  specimen.  These  four  characteristics  may 
be  considered  in  order. 


EPITHELIA.  27 


Origin. 


Epithelia  arise  from  all  three  of  the  germ  layers  as  described  in  the 
section  on  Histogenesis.  The  terms  which  may  be  applied  to  adult  epitheha 
indicating  their  origin  are  ectodermal,  entodermal,  mesodermal,  mesothelial, 
and  mesenchymal.  Mesothelium  is  a  term  applied  sometimes  to  all  meso- 
dermal epithelia  except  the  mesenchymal.  There  is  a  tendency,  however, 
which  seems  desirable,  to  limit  its  application  in  the  adult  to  the  pericardial, 
pleural,  and  peritonaeal  epithelia.  Endothelium  is  from  the  " angioblasi" 
and  lines  the  heart,  the  blood  vessels  and  the  lymphatic  vessels  only. 
The  loose  but  rather  common  application  of  this  name  to  mesothehum 
and  mesenchymal  epithelium  is  much  to  be  regretted.  Mesenchymal  [or 
false]  epithelia  are  formed  by  flattened  mesenchymal  cells,  developing 
relatively  late  in  embryonic  life.  They  line  the  biirsae,  tendon  sheaths, 
joint  cavities,  the  chambers  of  the  eye,  and  the  scalae  tympani  and  vestibuli 
of  the  ear.  The  table  on  page  26  indicates  to  which  germ  layer  the 
epithelia  belong. 

Shapes  of  Epithelial  Cells. 

Epithelial  cells  may  be  grouped,  according  to  their  shape,  in  three 
classes,  flat,  cuboidal,  and  columnar.    These  names  apply  to  the  appearance 


Fig.  24.— Amnion  of  Pig.    (A  Fetal  Membr.\ne  Covering  the  E.mbryo.) 
S.  C.  Epi.,  Simple  cuboidal  epithelium;  Mesen.,a  mesenchymal  tissue;  Meso.,  mesothelium,  a  simple 

flat  epithelium. 

of  the  cells  when  cut  in  a  plane  perpendicular  to  the  free  surface.  On 
surface  view  all  three  kinds  are  usually  polygonal  and  often  six  sided. 
If  the  epithelium  consists  of  but  a  single  layer  of  cells  it  is  called  simple. 
Fig.  24  shows  along  its  upper  surface  a  simple  cuboidal  epithelium.  ^  The 
sections  of  its  cells  are  approximately  square.  On  the  lower  surface  is  a 
simple  flat  epithelium,  which,  being  an  extension  of  the  layer  lining  the 
coelom,  is  a  mesothelium.  A  surface  view  of  mesothehal  cells  on  a  smaller 
scale  is  shown  in  Fig.  25,  A.  Endothelium,  Fig.  25,  B,  is  quite  like  meso- 
thelium in  appearance;  its  cells,  however,  are  usually  more  elongated, 
parallel  with  the  course  of  the  vessel  which  they  line.  It  is  a  simple  epi- 
thelium, so  flat  that  the  thickest  part  of  its  cell  is  that  which  accommodates 


28 


HISTOLOGY. 


the  nucleus.     In  Fig.  26  there  is  both  a  surface  view  and  a  section  of 

simple  columnar  epithelium.     Often  columnar  cells  are  nearly  cuboidal 

and  are  described  as  low  columnar. 

Gradations  between  all   the  types 

described  are  to  be  expected.    (The  , 

following  synonyms  are  in  common 


ri« 


Cuticula 


Cross  section 
of  a 
terminal  bar 


mm 

A  B 

Fig.  25.  Fig.  26. — Simple  Columnar   Epithelium  fro.m 

A,  Surface  view  of  niesotheliuni  from  the  mesen-  thk  Intestinal  Villus  of  Man. 

tery;  B,  surface  view  of  endollielium  from  an        A,  Surface  view;    B,  vertical  section.     The  promi- 
artery.  nent  cell  outlines  in  A  are  due  to  terminal  bars, 

shown  in  cross  section  at  the  left  of  B,  and  in 
de  view  at  the  right. 

use: — cylindrical   for   columnar;    pavement    for    cuboidal   or   flat;     and 
squamous,  meaning  scale-hke,  for  flat.) 

Number  of  Layers. 
A  simple  epithelium  may  be  so  arranged  that  it  appears  to  consist  of 
several  layers  (Fig,  27).     All  of  the  cells  start  from  the  connective  tissue 


Fig.  27. — Diagram  of  Pseudo- 
stratifihd  epithelium. 


I'iG.  28. — Stratified  Epi- 
thelium FROM  T  H  !•; 
Human  Larynx.  X  240. 

1,  Columnar  cells  ;  2,  poly- 
gonal cells;  3,  flat  (squa- 
mous) cells. 


Fig.  29.— Detached  Squa- 
MOL'S  Cells  from  the 
Mouth. 


below,  but  may  fail  to  reach  the  free  surface.     Their  nuclei  are  at  different 
levels..    Such  pseudo-sir alified  epithelium  is  found  in  parts  of  the  respira- 


EPITHELIA. 


29 


tory  and  genital  tracts.  A  stratijied  epithelium  is  one  which  actually 
consists  of  several  layers  of  cells  (Fig.  28).  In  descriptions  of  stratified 
epithelia  the  number  of  layers  should  be  recorded,  especially  if  few.  We 
may  say  that  it  is  2 -layered,  4-6-layered,  or  many  layered,  as  the  case  may 
be.  The  shapes  of  the  cells  in  the  basal,  middle,  and  superficial  strata 
should  be  noted.  The  cells  are  formed  in  the  basal  layer,  and  as  they  are 
pushed  outward  they  become  changed  in  shape  and  character.  The 
superficial  cells,  for  which  the  entire  stratified  epithelium  is  often  named, 
may  be  columnar,  cuboidal,  or  fiat.  The  fiat  ones  are  called  squamous, 
especially  when  they  have  become  detached  and  are  found  in  urine  or 
saliva  (Fig.  29).  (Transitional  epithelium  is  an  undesirable  name  for  that 
form  of  stratified  epithelium  found  in  the  bladder,  ureter,  and  pelvis  of 
the  kidney.     It  will  be  described  in  connection  with  those  organs.) 


Peripheral  Differentiation. 

The  differentiation  of  epithelial  cells  is  chiefly  along  three  lines, — 
first,  the  transformation  of  entire  cells  into  cornified  masses  as  in  the 
outer  cells  of  the  skin,  in  the  nails, 
and  hair;  second,  the  development 
of  various  structures  around  the 
borders  of  the  cells,  particularly 
along  the  free  surface;  and  third, 
the  elaboration  of  secretion  within 
the  protoplasm.  The  last  two  forms 
will  be  considered  in  detail. 

Cell  walls  in  young  epithelia 
are  generally  lacking.  In  the  early 
embryonic  skin  and  in  its  basal 
layers  in  the  adult,  they  are  often 
absent,  so  that  the  cells  are  in  very 
close  contact.  Later  they  become 
separated  from  one  another  by  "ce- 
ment substance,"  probably  fluid.  This  is  true  of  mesothehal  and  endo- 
thelial cells  also.  Since  silver  nitrate  is  precipitated  by  the  intercellular 
substance,  their  cell  boundaries  become  very  distinct  after  treatment  with 
this  reagent.  Lymph  corpuscles  and  leucocytes  may  pass  out  from  thin- 
walled  blood  vessels,  between  the  endothelial  cells,  into  the  mesenchymal 
spaces.  They  may  enter  the  intercellular  substance  between  the  columnar 
cells  of  the  intestinal  epithelium.  Here  they  are  prevented  from  reaching 
the  free   surface   by  terminal   bars.      The   diagram,  Fig.   30,  illustrates 


Network  of 
terminal 
bars. 


Cuticula.  /' 


Intercellular 
substance. 


Fig.  30. — DiAGR.\M  of  the  Network  of 
Terminal  Bars. 
rhe  two  cells  on  the  left  are  divided   lengthwise 
into  halves  ;  the  two  on  the  right  are  drawn  as 
complete  cylinders  or  prisms. 


30  HISTOLOGY. 

how  these  bars  encircle  each  cell  near  its  top,  binding  it  to  the  adjoin- 
ing cells.  The  bars  are  regarded  as  a  form  of  cement  substance.  In 
sections  of  the  intestine,  Fig.  26,  or  of  the  epididymis,  Fig.  33,  b,  they 
may  be  seen  with  ordinary  high  power  lenses.  Occasionally,  as  in  the 
deeper  layers  of  the  skin,  the  spaces  between  the  cells  are  crossed  by  delicate 
protoplsLsmic  bridges,  so  that  the  cells  have  a  spiny  appearance  (Fig.  31). 
Fine  fibrils  may  pass  from  cell  to  cell  through  these  bridges  which  are 
themselves  so  slender  as  scarcely  to  be  defined  without  oil  immersion 
objectives.  The  spaces  are  smaller  and  the' .bridges  shorter  in  simple 
than  in  stratified  epithelium.  Therefore  the  spaces  have  been  regarded 
as  canals  to  convey  nutriment  to  the  outer  cells.  Nutriment  comes  to 
epithelia  through  blood  vessels  in  the  tissue  just  beneath  them.  Except 
possibly  in  the  bladder  and  renal  pelvis  the  vessels  do  not  enter  an  epithe- 
lium, nor  are  lymphatic  vessels  found  within  it.  Whatever  nutriment 
the  outer  cells  receive  must  come  through  the  cells 
,    'Vv;''     ■  below  or  through  the  intercellular  spaces. 

f'V  '°  ^,  Intercellular   spaces   have   been   said   to   arise 

'      --:[\-  through  coalescence  of  vacuoles  in  the  exoplasm. 

'a^        .^,]i^-^  The  fact  that  the  spinous  cells,  with  intercellular  sub- 

stance between  them,  present  a  form  intermediate 
^'*^BRi'D^Fs^As'^sEEN''m     bctwccn  ordiuary  epithelium  and  mesenchyma  has 
OF  ^HE'^GERMmATivE     ^ccn  cmphaslzcd.    The  basal  cells  of  an  epithelium 
DERMfs.°''  ^"^  ^^^'     sometimes  seem  to  send  out  processes  which  con- 
nect with  the  underlying  mesenchymal  cells.      In 
glands  especially,  a  thin,  well-defined  membrane  is  often  found  just  under 
the  epithelium,  and  it  is  called  a  basement  membrane  (membrana  propria). 
It  is  usually  homogeneous  and  without  nuclei,  often  being  of  elastic  sub- 
stance.    Some  basement  membranes  are  held  to  be  formed  by  the  basal 
processes  of  epithelial  cells,  but  generally  they  are  considered  of  mesen- 
chymal derivation. 

Along  their  free  surface,  epithelial  cells  often  have  a  thick  wall  called  a 
cuticular  border  (top  plate,  or  if  very  thick,  a  cmsta).  Under  high  magnifi- 
cation some  cuticular  borders  appear  perpendicularly  striated  and  consist 
of  protoplasmic  processes  or  pseudopodia,  which  may  be  sent  out  or 
retracted,  thus  causing  the  border  to  vary  in  width.  This  has  been 
observed  in  the  human  large  intestine,  and  in  the  efferent  ducts  of  the  testis 
of  a  mouse,  Fig.  32,  a  and  b.  Longer  processes  which  are  vibratile  but  not 
retractile  are  known  as  cilia.  They  cover  the  free  surfaces  of  many 
epithelia  either  simple  or  stratified.  In^the  living  condition  the  motion 
of  cilia  may  be  observed  in  certain  unicellular  animals,  along  the  gills  of 
fresh  water  clams  or  in  pieces  of  oral  epithelium  from  a  frog.     The  stroke 


CILL4TED   EPITHELIA. 


31 


Fig.  32. — Cellsofthe  Efferent 
Ducts  of  the  Testis  of  a 
Mouse.    (After  Fuchs.) 

To  show  terminal  bars;  cuticular 
border  (in  b);  diplosomes;  and 
cilia  (in  c). 


of  cilia  is  effective  in  one  direction  only,  so  that  mucus  or  solid  particles 
may  be  swept  by  their  action  across  the  surface  of  the  epithelium,  for 
example  from  the  trachea  to  the  mouth.  In  the  lower  animals  the  stroke 
may  be  reversed  under  certain  conditions.  Or- 
dinarily the  student  can  merely  detect  the  pres- 
ence or  absence  of  ciha  in  a  given  specimen. 
Under  favorable  conditions  investigators  have 
observed  that  each  cilium  is  connected  with  a 
granule  or  pair  of  granules,  the  hasal  body, 
near  the  upper  surface  of  the  cell,  and  several 
agree  that  these  arise  by  division  of  the  cen- 
trosome.  In  Fig.  32,  a,  the  cell  contains  a 
single  diplosome  (centrosome)  in  characteristic 
position;  b  has  four  diplosomes;  and  c  is  cili- 
ated with  basal  bodies  similar  to  diplosomes. 
Apparently  no  ciliated  cell  has  been  obsen^ed 
in  mitosis.  Fig.  33,  a,  is  a  diagram  to  show  that  ciha  may  extend  through 
the  top  plate  into  the  protoplasm,  and  obscure  modifications  of  the  upper 
part  of  the  protoplasm  may  sometimes  be  seen  with  ordinary  magnifica- 
tion. The  row  of  diplosomes  may  appear  to  form  a  single  or  double 
transverse  line. 

The  cells  known  as  spermatozoa  are  each  provided  with  a  single, 
very  long,  motile  process,  such  as  is  called  a 
flagellum.  It  develops  in  relation  to  the  cen- 
trosome, as  vi-ill  be  described  in  connection  vv'ith 
the  testis.  Some  so- called  cilia  are  non-motile 
prolongations  of  the  filar  mass  of  the  proto- 
plasm, and  seem  to  be  concerned  with  the  dis- 
charge of  secretion.  They  have  no  basal  bodies 
and  lack  the  distinctness  of  true  ciha,  generally 
appearing  in  conical  clumps  like  the  hairs  of 
a  wet  paint  brush.  Such  cilia  are  found  in 
the  epididymis  (Fig.  33,  b).  In  certain  of  the 
kidney  ceUs  there  are  short,  thick,  non-motile 
processes,  described  sometimes  as  rudimentary 
cOia,  sometimes  as  a  cuticula,  and  known  as 
the  "brush  border."  The  cells  which  hne  the 
central  canal  of  the  nen-ous  system  develop 
processes  which  are  not  true  cilia.  Finally,  in  what  is  called  neuro-epi- 
thelium,  as  in  the  taste  buds,  the  epithelial  cells  have  one  or  more  slender 
processes  apparently  designed  to  receive  stimuli,  and  the  function  of  the 


Fig.  33. 
Diagram  of  a  ciliated  cell  (after 
Prenant),  showing  vibratile 
cilia  ;  b,  cells  of  the  human  epi- 
didymis (after  Fuchs),  showing 
ncn-motile  cilia. 


32 


HISTOLOGY, 


neuroepithelial  cell  is  to  transmit  this  stimulus  to  the  nerve  fibers  which 
branch  around  its  lower  part. 


Processes  of  Secretion. 

Manv  cells  can  elaborate  and  discharge  certain  substances  which  do 
not  become  parts  of  the  tissue.  Such  cells  are  called  gland  cells  and  their 
products  are  either  used  in  the  body  (secretions)  or,  being  of  no  value,  they 
are  removed  (excretions).  The  processes  of  elaboration  and  discharge 
of  either  secretion  or  excretion  may  often  be  recognized  by  changes  in 
the  form  and  contents  of  the  cell,  indicating  that  it  is  empty  or  full  of 
secretion,  as  the  case  may  be.  A  gland  cell  which  is  full  of  elaborated 
secretion  is  called  "active,"  and  one  in  which  the  secretion  is  not  apparent, 
though  it  may  be  in  process  of  formation,  is  called  "resting."  The 
appearances  of  secretion  difler  in  two  types  of  gland  cells,  the  serous,  which 
produce  a  watery  secretion  like  that  of  the  parotid  sahvary  gland,  and  the 


Granule. 


Protoplasm.    . J_5 


Basal  filaments. 
Nucleus. 


Granule. 


Protoplasm. 


New  s-ranule. 


Nucleolus. 


Fig.  34.— Two  Serovs  Gl.and-cells  fro.m  the  Sibm.^xill.ary  Gi.and  of  a  Glinea-pig.     X  1260. 

In  cell  B  tlie  granules  have  passed  into  the  unstainable  state  ;  new  stainable  granules  are  beginning 

to  develop  in  the  protoplasm. 

mucous,  which  form  a  thick  secretion  such  as  occurs  in  the  nose  and 
throat. 

The  nucleus  of  empty  gland  cells  often  has  a  fine  chromatin  network 
together  with  distinct  nuclear  granules.  The  granules  are  lacking  when 
the  cell  is  full  of  secretion  and  the  chromatin  takes  the  form  of  coarse 
fragments.  Doubtless  the  granules  pass  from  the  nucleus  into  the  proto- 
plasm, but  whether  they  become  true  secretion  granules  is  uncertain, 
since  similar  phenomena  have  been  observed  in  nerve  cells. 

The  protoplasm  of  serous  gland  cells  at  the  beginning  of  secretion 
exhibits  distinct  granules,  coarser  than  microsomes,  staining  intensely 
with  certain  dyes  (Fig  34,  A).  The  granules  enlarge,  lose  their  staining 
capacity,  and  are  transformed  into  drops  of  secretion  with  which  the  cell 
becomes  charged.    As  a  whole,  the  cell  is  larger  and  clearer  than  before. 


SECRETION. 


33 


P- 


Fig.  35. — Epithelial  Cells  Secreting  Mucus. 
From  a  section  of  the  mucous  membrane  of  the  stomach  of  man.  X  560. 
p,  Protoplasm  ;  s,  secretion;  a,  two  empty  cells;  the  cell  between 
them  shows  be.Efinning:  mucoid  metamorphosis  ;  e,  the  cell  on  the 
rig-ht  is  discharging  its  contents  ;  the  granular  protoplasm  has  in- 
creased and  the  nucleus  has  become  round  again. 


The  fluid  secretion  and  sometimes  the  granules  are  discharged  from  the 
free  surface.  ^ 

Such  cells  are 
striking  examples  of 
the  polarity  of  cells,  by 
which  is  meant  a  dif- 
ferentiation of  proto- 
plasm along  the  axis 
of  the  cell.  The  basal 
portion  receives  the 
nutriment  to  be  made  into  secretion.  It  often  exhibits  striations,  rods,  or 
filaments  known  as  ergasto plasm  (Fig.  34,  A).     The  distal  portion  which 

contains  the  elaborated  product 
is  obviously  of  a  different  nature. 
Very  many  kinds  of  cells  give  evi- 
dence of  polarity.  The  nuclear 
constituents  also  may  be  arranged 
in  relation  to  this  same  axis  or 
to  another,  but  nuclear  polarity 
which  is  manifest  during  mitosis 
may  be  disguised  or  lost  at  other 
times. 

In  mucous  cells  as  in  serous, 
secretion  begins  with  granule 
formation.  The  granules  soon 
change  into  clear  masses  of  mu- 
cus, which  accumulate  toward 
the  free  surface  and  are  more  or 
less  sharply  separated  from  the 
unchanged  protoplasm  beneath. 
The  mucus  is,  however,  pene- 
trated by  strands  of  protoplasm 
which  contain  the  centrosome.  As 
elaboration  of  mucus  continues 
the  nucleus  is  crowded  to  the  base 
of  the  cell,  and  may  become  round 
or  flattened  (Fig.  35).  Then  the 
secretion  is  gradually  discharged, 
apparently  with  the  rupture  of  the 
top  plate.  If  the  cell  is  not  de- 
stroyed the  nucleus  returns  to  its 


Secretion. 

Protoplasm 
and  nucleus. 


Gland  lumen. 


Fig.  36. — Intestin.al  Gland  from  a  Section  of 
THE  Large  Intestine  of  Man.    X  165. 

The  secretion  formed  in  the  goblet-cells  is  colored 
blue.  In  zone  i  the  globlet-cells  show  the  begin- 
ning of  secretion ;  that  expulsion  has  begun  is 
evident  from  the  presence  of  drops  of  secretion  in 
the  lumen  of  the  gland.  2,  Goblet-cells  with  much 
secretion.  3,  Goblet-cells  containing  less  secretion. 
4,  Dying  goblet-cells,  some  of  which  still  contain 
remnants  of  secretion. 


34  HISTOLOGY. 

central  position  and  the  protoplasm  relills  the  cell  now  greatly  reduced  in 
size.  Most  gland  cells  are  not  destroyed  by  the  discharge  of  secretion, 
but  may  repeat  the  process  several  times.  In  the  sebaceous  glands, 
however,  cells  and  secretion  are  cast  off  together,  and  many  of  the  mucus- 
producing  goblet  cells,  such  as  have  just  been  described,  are  thought  to 
perish  after  once  filling  with  secretion.  In  the  large  intestine,  goblet  cells 
are  formed  near  the  bottom  of  tubular  depressions  in  a  simple  columnar 
epithelium.  Fig.  36.  By  the  addition  of  new  cells  below  them,  they  are 
pushed  toward  the  outlet  of  the  tube  where  the  oldest  cells  are  found. 
Mucus  is  discharged  while  its  formation  continues.  For  a  time  the  secre- 
tion develops  faster  than  it  is  discharged,  so  that  it  accumulates  within  the 
cell  (Fig.  36,  2),  but  later,  as  elimination  exceeds  production,  the  cell 
becomes  emptied  and  dies  (Fig.  36,  4).  In  stratified  epithelium,  mucus 
may  be  formed  in  the  deeper  cells,  but  it  cannot  be  discharged  until  these 
have  reached  the  surface. 

The  Description  of  an  Epithelium. 
In  describing  an  epithelium  the  student  should  record  its  origin  if  it 
is  remembered,  and  should  note  from  observation,  first,  the  number  of 
layers  (whether  the  epithelium  is  simple  or  stratified;  in  the  latter  case, 
the  number  of  strata);  second,  the  shapes  of  the  cells  (columnar,  cuboidal, 
or  flat,  and  in  a  stratified  epithelium  the  layers,  basal  or  superficial,  in 
which  such  shapes  occur) ;  finally,  the  special  structures  should  be  sought, 
including  basement  membranes,  intercellular  bridges,  terminal  bars, 
striated,  brush,  or  ciliated  borders,  and  forms  of  secretion  within  the 
protoplasm.  A  detailed  description  of  nucleus  and  protoplasm  should 
be  given  of  such  epithelial  cells  as  are  of  special  importance. 

THE  NATURE  AND  CLASSIFICATION  OF  GLANDS. 

A  prehminary  description  of  glands  may  be  inserted  at  this  point, 
since  glands  in  the  strictest  sense  are  groups  of  such  secreting  epithehal 
cells  as  have  just  been  described.  Two  other  classes  of  structures  are  called 
glands,  however.  In  one  of  these,  cells  instead  of  secretions  are  formed 
and  set  free.  Cell-producing  glands  are  called  specifically  cytogenic 
glands.  These  include,  first,  the  ovary  and  testis  which  produce  sexual 
cells;  and,  second,  the  lymph  glands,  haemolymph  glands,  spleen,  and 
red  bone  marrow,  all  of  which  produce  blood  corpuscles.  Tissue  similar 
to  that  of  the  lymph  glands  when  found  in  a  diffuse  form  is  not  called  glan- 
dular, but  merely  lymphoid  tissue.  The  term  gland,  as  here  employed, 
suggests  a  well-defined,  macroscopic  mass  of  cell-producing  tissue,  epithe- 
lial in  the  sexual  glands,  and  non-epithelial  in  the  lymphoid  group. 


GLANDS. 


35 


Besides  the  cytogenic  glands,  there  are  epithelioid  glands  consisting 
of  clumps  or  cords  of  cells  resembling  epithelium,  yet  having  no  free  surface. 
These  masses  of  cells,  which  may  be  detached  from  an  epithelium  or 
formed  from  mesenchyma,  are  generally  penetrated  by  blood  or  lymphatic 
vessels  into  which  their  secretions  are  discharged.  Secretions  eliminated 
in  this  manner  are  called  internal  secretions.  The  epithelioid  glands  can 
produce  only  internal  secretions.  The  suprarenal  gland  is  a  large  example 
of  this  class. 

Epithelial  glands  are  such  as  consist  of  true  epithelium,  discharging 
their  secretions  from  its  free  surface.  Most  glands  are  of  this  nature. 
In  simplest  form  they  are  merely  the  occasional  mucous  or  other  secret- 
ing cells  found  scat- 
tered over  an  epithe- 
lium. These  are 
sometimes  called 
unicellular  glands. 
Others  are  simple 
tubular  or  saccular 
depressions  in  the 
epithelium,  lined 
with  secreting  cells 
as  shown  in  Fig.  36. 
Glands  of  this  de- 
scription, perhaps 
coiled  at  their  lower 
end,  or  having  a  few 
branches,  or  con- 
sisting of  a  cluster 
of  saccular  secret- 
ing spaces,  often 
occur  in  large  numbers  as  parts  of  some  organ.  Thus  they  are  found  in 
the  intestine,  the  uterus,  and  the  skin,  where  they  are  named  intestinal 
glands,  uterine  glands,  sebaceous  and  sweat  glands  respectively,  each 
kind  having  its  special  characteristics.  They  are  named  as  classes  and 
not  as  individuals,  and  have  been  grouped  as  the  simple  glands.  On  the 
other  hand,  there  are  epithelial  glands  which  occur  singly  or  in  circum- 
scribed groups,  having  their  own  connective  tissue  capsule,  blood,  nerve, 
and  lymph  supply.  Such  forms  are  considered  as  separate  organs,  for 
example,  the  hver,  pancreas,  mammary  gland,  and  prostate,  and  for  this 
group  the  name  compound  glands  has  been  introduced. 

These  glands  develop  in  the  embryo  generally  as  a  solid  downgrowth 


End  pieces. 


U'O 


Fig.  37. — Diagram  of  the  Develop.vient  of  a  Compound  Gland. 
The  arrangement  of  ducts  in  D  is  that  of  the  human  submaxillary  gland. 


36  HISTOLOGY. 

of  the  epithelium.  This  divides  by  branching,  and  subdivides  as  shown 
in  the  diagram,  Fig.  37,  A,  B,  and  C.  A  cavity  appears  in  the  cord  of  cells 
which  then  become  clearly  epithelial.  Simple  glands,  as  in  the  intestine, 
may  remain  in  the  stage  A,  and  be  lined  throughout  with  secreting  cells; 
in  glands  of  greater  size  and  complexity  only  the  terminal  portions  contain 
the  essential  secreting  cells.  The  tnmk  and  its  main  branches  serve  to 
convey  the  products  of  the  "end  pieces"  to  the  surface,  and  constitute  the 
ducts.  Stage  B  is  permanent  in  such  simple  glands  as  those  of  the  stomach, 
in  which  a  short  duct  without  branches  is  formed  by  the  union  of  a  few 
tubular  end  pieces.  The  compound  glands  generally  ha\'e  branching 
ducts  as  in  C  and  D. 

The  secreting  portions  of  the  gland  may  be  tubular,  spheroidal, 
or  of  some  intermediate  shape.     A  round  termination  is  called  either  an 

acinus  (Latin,  a  grape)  or  an  alveolus  (Latin, 

#a  small  rounded  vessel).  The  intermediate 
forms  are  called  alveolo-tiihular  [tubulo- 
acinar, etc.].  The  cavity  of  these  parts  is 
called  the  lumen  of  the  gland,  and  is  directly 
continuous  with  the  cavity  of  the  ducts. 
The  secretion  may  pass  from  the  cells  di- 
rectly into  the  gland  lumen,  or  it  may  enter 
extensions  of  the  lumen  found  either  between 
the  cells  or  actually  within  their  protoplasm. 
These  are  the  intercellular  and  intracellular 
Fig.  38.-D1AGRAM  OF  A  Simple  al-     sccrctory  capillarics  respectively.    Thev  may 

VEOLAR  Gland,  Showing  Inter-  -         •'  r  j  ,  j 

RfEs"- or'' Canals 'o™\'he'^r'ght:     ^^  branched  or  anastomosing,— that  is,  form- 
THE  [•^^^^^^'^^'-^''-'^'^  Canals  on     -^^g  networks  by  the  union  of  their  branches. 

The  intracellular  capillaries  have  less  distinct 
walls  than  the  others,  and  are  considered  transient  formations  related 
to  vacuoles.  The  diagram.  Fig.  38,  represents  one  half  of  a  simple  alveolar 
gland  with  intercellular  secretory  capillaries  on  the  right,  and  intracellular 
ones  on  the  left.  Both  kinds  are  found  in  the  sweat  glands,  the  liver, 
and  the  gastric  glands.  Intercellular  capillaries  only  are  found  in  the 
serous  glands  of  the  tongue  and  in  the  serous  portions  of  the  salivary  glands, 
also  in  the  bulbo-urethral,  pyloric  and  lachrymal  glands.  Secretory  capil- 
laries are  apparently  absent  from  mucous,  duodenal,  intestinal,  uterine 
and  thyreoid  glands,  and  from  the  kidney  and  hypophysis. 

Having  reached  the  gland  lumen,  the  secretion  may  pass  into  a  narrow 
duct  lined  with  simple  cuboidal  or  fiat  epithelium,  the  intercalated  duel 
of  Fig.  37,  D.  The  transition  from  this  to  the  larger  duct,  lined  perhaps 
with  columnar  epithelium,  is  not  as  abrupt  as  in  the  diagram.     In  certain 


EPITHELIAL    GLAXDS.  37 

glands  the  cells  here  show  basal  striations,  due  to  rows  of  granules,  which 
indicate  that  this  portion  of  the  ducts  produces  a  secretion.  The  terminal 
part  of  the  ducts  of  a  large  gland  may  be  formed  of  stratified  epithehum, 
perhaps  containing  mucous  cells.  The  ducts  of  the  liver  produce  a 
considerable  quantity  of  mucus,  and  the  bronchi,  which  from  their  develop- 
ment and  form  may  be  considered  the  ducts  of  the  lungs,  also  contain 
scattered  mucous  cells  and  small  secondar}'  mucous  glands.  Important 
secretions  are  elaborated  by  the  efferent  and  some  other  ducts  of  the  testis. 
In  the  kidney  there  is  no  terminal  secreting  portion  as  in  most  glands. 
The  duct-like  tubules  serve  rather  to  transfer  selected  materials  from  the 
blood  to  the  lumen  than  to  form  new  substances.  This  is  more  obviously 
true  of  the  alveoli  of  the  lung  which  merely  transmit  oxygen  and  other 
substances  through  inert  modified  cells.  Morphologically,  that  is,  in  their 
form  and  development,  both  the  kidneys  and  the  lungs  are  glands. 

All  epithelial  glands  arise  as  outgrowths  from  an  epithehum,  as  has 
been  described.  A  few,  by  the  obliteration  of  their  ducts,  become  separated 
from  their  place  of  origin.  This  occurs  in  some  small  glands  associated 
with  the  brain  and  in  the  thyreoid  gland.  The  closed  end  pieces  of  the 
thyreoid  become  filled  with  a  secretion  that  cannot  escape.  Derived  from 
or  in  addition  to  this,  there  is  an  internal  secretion  which  is  taken  up  by 
the  vessels  adjacent  to  the  basal  surfaces  of  the  cells. 

.  For  completely  closed  epithelial  sacs,  such  as  occur  in  the  thyreoid 
gland  and  in  the  ovary,  the  term  jollide  is  used  (Latin,  jolliculus,  "a  Uttle 
bag").  If  such  closed  spaces  are  pathological  or  degenerative,  they  are 
called  cysts.  Small  round  solid  masses  of  lymphoid  tissue,  occurring 
singly  or  as  parts  of  lymph  glands,  are  called  nodules  (Latin,  nodulus, 
"a  little  knot").  Very  often  and  improperly  lymph  nodules  are  called 
follicles. 

In  examining  sections  of  glands  the  student  should  observe  to  what 
class  they  belong,  and  should  record  in  case  of  epithelial  glands  whether 
they  are  unbranched  or  branched,  together  with  the  shape  of  the  end  pieces. 
It  is  often  difficult  to  determine  this  shape  without  resort  to  reconstructions 
from  a  series  of  sections.  The  various  appearances  of  the  ducts  should 
be  studied  with  the  idea  of  picturing  the  gland  as  a  whole. 

As  a  summary  of  the  preceding  paragraphs  the  following  tabular 
classification  of  glands  may  be  presented: 

I.  Epithelial  glands,  with  persistent  ducts,  producing  external  secre- 
tions. 

1.  Unicellular  glands. 

2.  Simple  glands. 

a.  Ectodermal,  e.  g.,  sweat  and  sebaceous  glands. 


38  HISTOLOGY. 

h.  Mesodermal,  e.  g.,  uterine  glands. 
c.  Entodermal,  e.  g.,  gastric  and  intestinal  glands. 
3.  Compound  glands. 

a.  Ectodermal,  e.  g.,  mammary  and  lachrymal  glands. 

b.  Mesodermal,  e.  g.,  epididymis  and  kidney. 

c.  Entodermal,  e.  g.,  pancreas  and  liver. 

II.  Epithelial  glands,  with  obliterated  ducts,  producing  internal  secretion. 

a.  Ectodermal,  pineal  body ;  both  lobes  of  the  hiq^ophysis. 

b.  Entodermal,  thyreoid  gland. 

III.  Epithelioid  glands,  never  having  duct  or  lumen,  producing  internal 
secretions  only. 

a.  Ectodermal    (through    their    relation    to    the    sympathetic 

nerves),  glomus  caroticum;  glomus  coccygeum;  and  me- 
dulla of  the  suprarenal  gland. 

b.  Mesodermal,  cortex  of  suprarenal  gland;  interstitial  cells  of 

the  testis;   corpus  luteum. 

c.  Entodermal,  islands  of  the  pancreas;   epithelioid  bodies  in 

relation  with  the  thyreoid  gland;  thymus  in  its  early  stages. 

IV.  Cytogenic  glands,  producing  cells. 

a.  Mesodermal,  epithelial,— the  ovary  and  testis. 

b.  Mesodermal,  mesenchymal, — the  lymph  glands,  haemolymph 

glands,  spleen,  red  bone  marrow,  and  many  smaller  struc- 
tures. 

THE  MESENCHYMAL  TISSUES. 

In  an  early  stage  the  embryo  is  composed  of  two  tissues,  epithelium 
and  mesenchyma.  Mesenchyma  has  already  been  defined  as  a  non-epithelial 
portion  of  the  mesoderm  composed  of  branching  cells.  Their  protoplasmic 
processes  anastomose,  forming  a  continuous  network  of  protoplasm, — a 
syncytium,  in  the  meshes  of  which  is  a  homogeneous  intercellular  substance 
or  matrix  (Fig.  22,  page  23).  Those  derivatives  of  mesenchyma  which 
diverge  greatly  from  this  embryonic  type  will  be  reserved  for  later  considera- 
tion. Such  are  the  vascular  systems,  smooth  muscle  and  certain  epithelioid 
cells.  Reticular  tissue,  mucous  tissue,  connective  tissue,  tendon,  cartilage 
and  bone,  sometimes  grouped  as  the  supporting  tissues,  may  now  be  con- 
sidered in  turn.  They  arc  all  mesenchymal  tissues  which  have  undergone 
transformations  both  of  their  cells  and  of  the  intercellular  substance. 

Reticular  Tissue. 

Reticular  tissue  is  that  form  of  adult  tissue  which  most  closely  re- 
sembles mesenchvma.     It  is  a  network  of  cells  with  a  fluid  intercellular 


RETICULAR   TISSUE.  39 

substance.     The  protoplasmic  processes,  however,  have  been  transformed 
into   stiff   slender  fibrils   containing   a   substance   known   chemically   as 
reticulin.     Whereas   ordinary  connective  tissue  may  be  made   to  yield 
gelatin,  reticular  tissue  gives  both  gelatin  and  reticulin.     Since  connec  ive 
and  reticular  tissues  occur  so  closely  associated  that  it  would  be  difficult 
to  obtain  pure  specimens  of  the  latter,  the  gelatin  has  been  ascribed  to 
a  mixture  with  connective  tissue  elements.     On  the  other  hand,  it  has  been 
asserted  that  reticulin  is  merely  a  variety  of  gelatin  due  to  the  method  of 
analysis.     Reticular    fibers,    by    their    greater    resistance    to    pancreatic 
digestion  and  by  dissolving  in  dilute  acid,  differ  from  the  elastic  elements 
of  connective  tissue.     They  are  said  to  be  more  resistant  to  acids  or  alkalies 
than  the  fibrillar  part  of  connective  tissue.     Such  a  distinction  is  hard  to 
establish,  especially  since  some  reticular  tissues  are  more  resistant  than 
others.     Chemically,  therefore,  the  validity  of  reticulin 
is  questionable.     Histologically  reticular  tissue  is  quite 
clearly  defined  by  the  abundance  and  fluidity  of  its 
matrix.      Small  round  cells,  the  lymphocytes,  which 
may  be  scattered  through  ordinary  connective  tissue,       |§  i   /^s^^      •  J 
are  always  abundant  in  the  meshes  of  reticular  tissue.       p.;L;     (''^-.J - 
They  are  so  numerous  and  closely  packed  that  the      T  H  H-/       %; 
delicate  reticular  fibers  are  mostly  hidden,  and  can  be       V"C    U        fl 
studied  to  advantage  only  after  the  loose  cells  have      '  V..,^^£^ 
been  disengaged  from  their  meshes.     This  may  be  ac-       A,    /^       -M 
complished  by  shaking  or  brushing  the  sections,  or  by  '"     ■'  '-"'■  '   '^ 

artificially  digesting  the  specimen  (which  destroys  the  fig.  39.— reticular 

^         "  "  ^  ^  ^  Tissue    from     the 

reticular  cells  along  with  the  others,  but  leaves  the  fibers)        spleen  of  the  pig. 

..  n.,  Nucleus;     f.,  fiber,   of 

or  by  the  method  of  Prof.  Mall,  used  m  obtammg  Fig.  ^g.        reticulin ;  \.  s.,  inter- 

\  '         _  7     .    ^   .  cellular  space. 

A  piece  of  fresh  spleen  was  distended  by  injecting 
gelatin  into  its  substance ;  then  frozen  and  sectioned.  The  sections  were 
put  in  warm  water  which  dissolved  out  the  gelatin,  carrying  the  loose 
cells  with  it,  and  leaving  areas  of  clear  reticular  tissue.  In  ordinary  sec- 
tions the  student  will  recognize  reticular  tissue  by  the  cells  in  its  meshes, 
but  some  of  its  nuclei  and  fibers  can  always  be  detected  upon  close  ex- 
amination. It  may  contain  cells  other  than  lymphocytes,  for  it  forms  the 
framework  not  only  of  lymph  glands,  but  of  red  bone  marrow  and  the 
spleen.  A  layer  of  reticular  tissue  is  found  under  the  epithelium  of  the 
digestive  tract,  and  it  has  been  reported  in  many  organs. 

Mucous  Tissue.  ,■  ^,^,^  ,^^  [^^^ ,  u.;  j 

Mucous  tissue  forms  the  substance  of  the  umbihcal  cord,  where  it 
was  formerly  called  Wharton's  jelly.     There  it  occurs  as  a  gelatinous 


40 


HISTOLOGY. 


after  they  have  left  the  cells. 


d.f. 


m.    V     i.i. 


tissue  of  pearly  luster,  containing  neither  capillary  nor  lymphatic  vessels, 
nor  nerves.  In  the  umbilical  cords  of  young  embr\-os  it  closely  resembles 
mesenchyma.  At  birth  its  cells,  which  retain  their  protoplasmic  con- 
nections with  one  another,  appear  fusiform  (spindle-shaped)  or  triangular 
rather  than  stellate.  The  intercellular  substance  consists  of  fibrils  in 
irregular  bundles,  embedded  in  a  matrix  containing  mucus.  It  has  long 
been  debated  whether  these  fibrils  originate  in  the  matrLx  directly,  by  a 
sort  of  precipitation  or  coagulation,  or  develop  in  the  outer  protoplasm 
(exoplasm)  from  which  they  later  become  separated.  The  tendency  is 
toward  the  latter  interpretation.  In  specimens  specially  stained,  Fig.  40, 
the  protoplasm  may  present  a  sharp  fibril-like  border  staining  differently 
from  the  intercellular  fibrils.     Chemical  changes  in  the  fibrils  may  occur 

Elastic  fibers,  to  be  described  under  con- 
nective tissue,  are  not  found  in  the 
mucous  tissue  of  the  umbilical  cord. 

The  mucins  are  a  group  of  com- 
pound proteid  bodies  containing  a  car- 
bohydrate complex  in  their  molecule 
and  therefore  known  as  glycoproteids. 
There  are  many  varieties,  the  mucus 
of  gland  cells  and  of  the  mucous  tissue 
just  described  both  containing  true 
mucins.  Related  substances,  called 
mucoids,  have  been  obtained  from  ten- 
don, cartilage  and  bone.  The  develop- 
ment of  mucus  by  connective  tissue 
cells  does  not  produce  anything  corre- 
sponding with  goblet  cells.  It  is  only 
in  connection  with  other  sorts  of  secretion  that  connective  tissue  cells  are 
said  to  elaborate  granules  which  are  converted  into  vacuoles. 

All  embryonic  connective  tissues  are  thought  to  contain  mucus,  and 
a  variety  of  tumor  (myxoma)  is  of  this  tj'pe.  The  pecuhar  connective 
tissue  of  the  cornea,  to  be  described  in  connection  with  the  eye,  contains 
no  elastic  fibers  and  is  rich  in  mucin;  nevertheless  its  structure  is  very 
different  from  that  of  the  substance  of  the  umbilical  cord,  to  which  the 
name  "mucous  tissue"  is  particulary  applicable. 


— Mucous  Tissue  from  the  Human 
Cord,  at  Birth.    Mallory's  con- 


.  f    Di^»fcOuiBdle  of  fibrils;    m.,  mucus — con- 
"■T^W^^  iotercellular  substance;    l.f.,   loose 
fibrils;  c,  cell  with  fibril-like  border. 


Connective  Tissue. 

Connective  tissue  is  that  derivative  of  mesenchyma  which  consists  of 
cells  either  connected  with  one  another  or  disconnected,  and  of  intercellular 
spaces  largely  occupied  by  fibers  of  two  sorts,  white  and  elastic  fibers 


COXXECTIVE    TISSUE. 


41 


respectively.  In  the  dense  forms  of  connective  tissue  the  fiber- bundles 
tend  to  be  parallel  and  are  closely  packed.  In  loose  or  areolar  connective 
tissue  the  fibers  run  in  various  directions,  and  among  them  are  cells  which 
have  become  charged  with  fat.  When  these  are  numerous  they  constitute 
fat  tissue  (adipose  tissue).  Areolar  connective  tissue  ordinarily  contains 
fat  cells.  In  every  specimen  of  connective  tissue  three  features  should 
be  examined:  the  fibers,  the  cells,  and  the  remains  of  the  intercellular 
substance. 

Fibers.  If  a  small  piece  of  fresh  connective  tissue,  such  as  envelops 
the  muscles  of  a  guinea  pig,  be  pulled  apart  on  a  shde  and  examined  in 
water,  it  will  exhibit  the  structures  shovvii  in  Fig.  41.  Most  of  the  specimen 
may  be  obscure,  but  in  such  parts  as  were  properly  spread  out  the  ivhite 
fibers  can  be  seen  as  pale,  wavy 
bands,  without  sharp  borders. 
They  are  faintly  striated  longi- 
tudinally, due  to  the  fact  that 
they  are  bundles  of  minute 
fibrils  bound  together  by  a 
small  amount  of  cement  sub- 
stance. The  addition  of  picric 
acid  causes  them  to  separate 
into  their  constituent  elements. 
The  white  fibers  divide,  as 
shown  in  the  figure,  by  the 
separation  of  the  fibrils  into 
smaller  groups ;  the  fibrils 
themselves  do  not  branch.  If 
dilute  acetic  acid  is  put  upon 
the  specimen,  these  fibers  swell, 
as  shoTVTi  in  Fig.  41,  B,  often  presenting  a  series  of  constrictions  ascribed 
to  the  remains  of  encircling  ceUs,  to  rings  of  elastic  fiber,  or  to  remnants  of 
a  sheath  which  enveloped  the  bundle.  Ultimately  the  white  fibers  disap- 
pear in  acids  or  in  alkalies.  Chemically  they  are  said  to  consist  of  collagen, 
an  albuminoid  body  which  on  boiling  yields  gelatm  (glutin,  the  source  of 
glue).  The  white  fibers  are  supposed  to  arise  in  the  exoplasm.  Those 
seen  in  mucous  tissue  were  of  this  variety. 

Elastic  fibers  are  probably  always  present  in  connective  tissue,  though 
varying  greatly  in  their  abundance.  They  are  said  to  develop  later  than 
the  white  fibers  and  are  absent  from  corneal  tissue,  mucous  tissue,  and 
generally,  though  not  always,  from  reticular  tissue.  In  Fig.  41  they  are 
seen  as  sharply  defined,  straight  or  stiffly  bent,  homogeneous  structures 


Fat  CeU 


Wh-E 


Fig.  41. — Fresh  Connective  Tissue  from  Around  the 
Shoulder  Muscles  of  a  Guinea  Pig. 

A,  Before  and  B.  after  adding  dilute  acetic  acid.  El.  F.,  Elas- 
tic fiber;  Wh.F.,  white  fiber;  n.,  nucleus  of  connective 
tissue  cell. 


42 


HISTOLOGY. 


which  are  highly  refractive, — that  is,  they  so  reflect  light  as  to  change  from 
bright  to  very  dark  objects  on  varying  the  focus.  They  may  be  extremely 
fine,  or  quite  broad,  but  the  latter  are  not  divisible  into  smaller  elements 

or  fibrils.  Seen  in  specimens  which  have 
not  been  torn  apart,  the  elastic  fibers  form  a 
network,  Fig.  42,  A,  and  the  smooth  manner 
in  which  they  fuse  at  its  angles  is  charac- 
teristic. When  the  net  is  broken  the  fibers 
retract  in  irregular  spirals.  The  elastic 
fibers  are  thought  to  be  of  exoplasmic  origin, 
as  is  suggested  by  Fig.  42,  B.  Elastic  sub- 
stance may  appear  within  the  cell  as  fila- 
ments, or  as  granules  which  later  fuse.  In 
some  cases  the  fibers  forming  the  elastic  net 
are  wider  than  its  apertures,  as  shown  in 
the  lower  part  of  Fig.  43,  A.  Here  they 
constitute  a  perforated  elastic  plate,  called  a 
fenestrated  membrane,  and  such  occur  in 
many  blood  vessels.  B  and  C  of  the  same  figure  present  elastic  elements 
from  the  ligame?itum  nuchae,  a  structure  containing  relatively  little  white 


Fig.  42. 
A,  Elastic  fibers  of  the  subcutaneous 
areolar  tissue  of  a  rabbit.  (After 
Schafer.)  B,  Cells  in  relation  with 
elastic  fibers,  after  treatment  with 
acetic  acid.  Subcutaneous  tissue  of 
a  fetal  pig.     (After  Mall.) 


■':^Qs>i 


<^ 


Fig.  43.— Elastic  Fibers. 
A,  Network  of  thick  fibers  below,  passing  into  a  fenestrated  membrane  above.     (From  the  endocardium 
of   man.)     B,  Thick  elastic   fibers,   f,  from    the   lixamentuni    nuchae    of    the    ox;    b,   white    fibers. 
C,  Cross  section  of  the  ligamentum  nuchae,  lettered  as  in  B. 


fiber,  and  hence  used  for  the  chemical  analysis  of  elastic   fiber.     The 
stylo-hyoid  ligament  and  the  ligamenta  flava  are  also  elastic  ligaments. 

Elastic  fibers  are  not  destroyed  by  dilute  acids  (Fig.  41,  B)  or  alkalies. 
They  consist  of  elastin,  an  albuminoid  body  which  does  not  yield  gelatin 


FAT    CELLS.  43 

on  boiling.  Because  of  the  difference  in  chemical  composition,  elastic 
fibers  may  be  stained  with  dyes  which  fail  to  color  white  fibers:  thus 
resorcin-fuchsin  stains  them  dark  purple,  but  scarcely  affects  the  white 
fibers;  on  the  other  hand,  Mallory's  connective  tissue  stain  makes  the 
white  fibers  deep  blue,  the  elastic  elements  remaining  colorless  or  pale 
pink.  These  special  stains  are  of  the  greatest  importance  in  studying 
connective  tissue.  In  ordinary  specimens  white  fibers  appear  blended 
in  masses  and  the  small  elastic  fibers  are  invisible.  There  may  be  other 
sorts  of  fibers  than  the  white  and  elastic,  such  as  the  fibroglia  of  Prof. 
Mallory,  but  these  are  still  very  little  understood. 

Cells.  Usually  the  cells  of  connective  tissue  are  conspicuous  only 
through  their  flattened  nuclei,  which  are  broadly  elliptical  on  surface 
view,  and  rod  shaped  when  seen  on  edge.  The  protoplasm  forms  a  wide, 
thin  layer,  and  since  it  is  closely  applied  to  the  fiber  bundles  which  it  may 
encircle,  and  ordinarily  stains  like  them,  very 
often  it  can  scarcely  be  distinguished.     x\s  a  ■,    ^"^.......^ 


.    fv 

whole,  the  cells  are  irregularly  polygonal,  flat-  |  /  '%,  -^ 

tened,  and  bent  to  conform  with  the  fibers.    In  ^^  \  Hv'C^. 

some  lamellar  tissues  these  flat  cells  are  in  con-  .,  ,^^'  ■■■..  ^  /^ 

tact  with  one  another  along  their  edges,  thus  |  {'^    :     '    'v  ^ 

simulating  an  epithelium.     In  loose  connective  '-^fri' '■  ^.  v|),ct 

tissue  they  may  be  widely  separated.      They  &^'  ■■  ■    -     -"'  ^ 
possess  processes  which  may  or  may  not  unite 

.  ,       ,  p  ,  ,,  1    •       ,1      •  ,  Fig.    44. — Developing    Subcuta- 

with  those  from  other  ceils,  and  m  their  proto-  neous  fat  cells,    hu.man 

.  Fetus  of  Five  Months. 

plasmic  bodies  there  are  often  a  few  small  fat     n., Nucleus;  f.v.,fat  vacuole;  p.  r., 

.,  ,  protoplasmic  rim. 

droplets. 

Fat  cells,  as  may  be  seen  in  the  subcutaneous  tissue  of  a  five  months' 
fetus  (Fig.  44)  arise  from  mesenchymal  cells  by  the  development  of 
vacuoles  of  fat  within  their  protoplasm.  The  vacuoles  enlarge  and  coalesce, 
so  that  the  nucleus  is  crowded  to  one  side,  lying  in  a  rim  of  unaltered 
protoplasm.  Gradually  the  protoplasmic  processes  disappear.  The  re- 
sulting form  of  cell  has  often  been  compared  with  a  "signet  ring,"  referring 
to  its  appearance  when  seen  in  section.  The  vacuole  of  fat  further  enlarges 
so  that  the  nucleus  is  flattened  and  the  protoplasmic  layer  becomes  very 
thin.  In  fresh  cells  it  cannot  be  seen.  The  entire  structure  appears  as  a 
large  refractive  drop  of  oil,  Fig.  41,  spheroidal  if  occurring  singly,  or 
polyhedral  if  compressed  by  adjoining  cells.  Small  fat  drops  may  be 
scattered  through  the  specimen  due  to  rupture  of  the  cells.  In  order  to 
study  fat  in  sections  it  is  necessary  to  employ  special  reagents.  The  tissue 
may  be  preserved  either  in  osmic  acid  which  blackens  the  fat,  or  in  a 
formalin  solution  and  afterwards  stained  with  Sudan  III  or  Sharlach  R, 


44 


HISTOLOGY. 


which  color  the  fat  droplets  red  and  demonstrate  them  even  when  minute. 
In  ordinary  sections  all  the  fat  has  been  dissolved  by  treatment  with  alcohol, 
leaving  the  protoplasmic  rims  enclosing  empty  spaces.  The  spaces, 
however,  correspond  in  size  and  shape  with  the  droplets  of  fat  which  have 


Surface  view  of  fat  cells,  in  the  nuclei  of  which  fat  droplets  are  visible. 


Connective  tissue     Blood  vessel  contain- 
cells.  ing  corpuscles. 


Fat  cell  and  its  nucleus  in  side  view.  Blood  capillary.  Connective  tissue. 

Fig.  45.— Fat  Tissue  from  the  Human  Scalp. 

been  removed.  Provided  that  the  cells  have  not  collapsed,  they  appear 
as  large,  round  or  polygonal  structures  (Fig.  45).  Some  are  seen  in 
surface  view,  as  if  looked  down  upon,  and  may  show  a  broadly  elliptical 
nucleus  containing  perhaps  one  or  two  small  vacuoles.  Most  of  the  cells 
in  thin  sections  are  cut  across.     The  protoplasmic  rim,  reduced  to  a  line, 

may  be  seen  to  widen  and  enclose  the  nucleus, 
but  often  no  nucleus  is  found.  This  is  because 
the  fat  cells  are  so  large  that  they  may  be  cut 
into  several  slices,  only  one  of  which  carries 
with  it  the  nucleus.  Filling  the  spaces  between 
the  cells  there  is  more  or  less  connective  tissue 
containing  blood  vessels.  The  student  should 
distinguish  the  nuclei  within  the  fat  cells  from 
such  connective  tissue  nuclei  as  may  be  closely 
adjacent  to  them.  In  some  sections,  radiating 
slender  crystals,  often  ill  defined,  will  be  seen 
within  the  fat  vacuole.  These  are  fat  crystals 
[margarin  crystals]  which  formed  as  the  fat 
cooled  and  solidified;  in  the  living  body  fat  is  fluid. 

All  fat  cells  do  not  contain  a  single  large  vacuole.     As  described  by 
Dr.  H.  A.  Christian  there  occur  both  at  birth  and  in  the  adult   such  fat 


Fig.  46. — Fat  Cells  from  Near 
THE  Kidney  of  a  New-born 
Child. 


CONNECTIVE   CELLS.  45 

cells  as  are  dra^vn  in  Fig.  46.  Their  protoplasm  contains  a  number  of 
large  vacuoles  and  the  nucleus  is  sometimes  central.  Such  cells  may  be 
found  in  subcutaneous  tissue,  but  are  more  often  seen  in  the  omentum 
or  around  the  kidneys.  In  extreme  emaciation  the  fat  cells  become 
flattened  and  several  small  vacuoles  replace  the  one  large  one.  These 
cells  are  said  to  produce  a  mucoid  substance  appearing  both  between  and 
in  the  cells. 

Fat  cells  develop  in  the  fetus  in  lobular  groups  around  small  blood  vessels. 
They  are  always  found  under  the  skin,  behind  the  eye  and  in  other  definite  places, 
so  that  they  have  been  regarded  as  secretory  organs.  Like  gland  cells  they  take 
material  from  the  vessels  near  by,  either  fat  which  is  stored  with  but  little  change, 
or  sugar  and  probably  albuminoid  bodies  which  are  transformed  into  fat  by  the 
activities  of  the  cell.  The  process  has  been  said  to  begin  in  or  near  the  nucleus 
with  the  formation  of  granules,  which  disappear  as  the  vacuoles  develop  around 
them.  The  small  vacuoles  in  the  nucleus  have  been 
described  as  containing  an  alkaline  fluid  which  is 
not  fat,  and  which  is  discharged  into  the  proto- 
plasm. They  are  also  described  as  fat  droplets  and 
are  observed  in  cells  full  of  fat  rather  than  in  those 
beginning  its  formation.  Like  an  internal  secre- 
tion, fat  is  taken  from  the  cells  into  the  vessels, 
though  probably  not  in  the  form  in  which  it  is 
stored.  It  should  be  remembered,  however,  that 
most  ceUs  take  material  from  the  blood  and  trans- 
form it  into  new  substances.  They  also  very  gen- 
erally may  effect  the  body  by  the  products  of  their 
activity.  Unless  the  term  "gland  cell"  is  to  be  so 
extended  as  to  lose  its  significance,  lobules  of  fat 
should  not  be  considered  glands.  Fig.  47  — fat  Cells  from  the 

"  Axilla    of    an    Extremely 

Emaciated    Individual. 

Besides  the  mesenchymal  cells  which  early  <  ^40. 

,  ,.„  .  ,    .  .  n         1  Ti  1-         k,Xucleus;  f,  fat  droplets ;  c,  cap- 

become  ainerentiated  into  tat  cells,  the  cells  or  iiiarybiood  vessels  ;b,  connec- 

tive tissue.. 

adult  connective  tissue,  of  cartilage,  and  the 

epithelium  of  the  liver  all  form  fat  vacuoles  which  may  or  may  not  coalesce. 
Pathologically  fat  appears  in  many  kinds  of  cells,  sometimes  representing 
an  accumulation  of  nutrient  material  which  the  cells  are  unable  to  as- 
similate, sometimes  resulting  from  the  breaking  down  of  the  normal 
combined  fats  into  vacuoles  of  free  fat.  It  is  customary  to  speak  of  such 
cells  as  "fatty  liver  cells,"  "cartilage  cells  containing  fat,"  etc.,  and  to  restrict 
the  term  ''fat  cell"  to  those  of  mesenchymal  origin  distended  with  one  or  a 
few  large  vacuoles. 

Pigment  cells  are  cells  of  mesenchymal  type  the  protoplasm  of  which 
contains  colored  granules.  The  granules,  which  are  generally  unaffected 
by  stains,  appear  brown  or  black  in  sections,  and  are  composed  of  melanin 
in  some  of  its  various  forms.  The  changes  of  color  in  the  chameleon 
are  largely  due  to  the  contraction  or  extension  of  the  processes  of  such 


46 


HISTOLOGY. 


pigment  cells.  In  man  -this  type  of  cell  is  of  limited  occurrence,  being 
found  chiefly  around  the  eye  (Fig.  48,  A).  The  same  sort  of  pigment 
may  be  found  in  epithelial  cells.  Thus  it  appears  in  the  epithelium  of 
that  part  of  the  conjuncti^"a  which  covers  the  bulb  of  the  eye  in  the  guinea 
pig  (Fig.  48,  B),  and  as  has  recently  been  noted,  it  occurs  there  in  all  human 
races  but  the  European.  The  pigment  of  the  skin  in  the  negro  races  and 
of  the  nipple  in  others  is  of  this  sort.  It  has  been  discussed  whether  such 
pigment  arose  in  epithelial  cells  or  was  transferred  to  them  from  underlying 
connecti^-e  tissue  cells,  or  actually  remained  in  such  underlying  cells 
(Fig.  48,  C).  The  retina  affords  positive  evidence  that  pigment  may 
develop  in  epithelial  cells,  and  it  has  even  been  said  that  some  of  these 
become  detached  and  send  out  branches.  The  term  "pigment  cell"  as 
ordinarily  used  refers  to  a  branched  cell  of  mesenchymal  origin.  Others 
are  said  to  "contain  pigment  granules,"  or  to  be  "pigmented  epithehal  cells." 
Finally,  it  should  be  added  that  the  melanin  series  of  pigments  is  one  of 
three  which  give  color  to  the  body.  The  others  are  the  fat  pigments,  or 
lipochromes,  and  the  blood  pigments,  or  haemoglobin  derivatives.  Cells 
containing  these  other  pigments  are  seldom  called  pigment  cells. 


^:> 


sg. 


<^      -c.t. 


A 


C 


Fig.  4S. 
A,  Two  pigment  cells  from  the  deep,  peripheral  part  of  the  cornea  of  the  rabbit.     B,  Pigmented  epithelium 
from  the  conjunctiva  of  the  guinea  pig.     The  pigment  is  chiefly  in  the  basal  layer.     C,  Pigment  cells 
sending  processes  between  the  epithelial   cells  of  the   skin  of  an   embryo  lizard,  Lacerta.     (After 
Prenant.) 


Besides  the  pigment  cells,  fat  cells,  and  fiber-producing  cells  {fihro- 
hlasts)  several  other  forms  occur  in  the  meshes  of  connective  tissue.  These 
are  free  from  one  another  and  are  merely  lodged  in  the  connective  tissue 
meshes.  Some  of  these  cells  emigrate  from  the  blood  vessels  in  adult  life. 
Others  may  be  descendants  of  cells  which  emigrated  from  the  vessels  in 
the  young  embryo,  or  else  they  may  have  arisen  directly  from  mesenchyma 
in  the  neighborhood  of  the  vessels.  A  more  definite  statemxcnt  concerning 
them  is  not  justified.  The  free  cells  in  connective  tissue  have  been  recently 
classed  as  lymphocytes,  plasma  cells,  '^resting  wandering  cells, ^^  mast  cells, 
and  eosinophiles.  All  of  these  types  except  the  resting  wandering  cells 
are  well  known  and  generally  recognized. 


THE    CELLS    IX    COXXECTRT:    TISSUE. 


47 


Lymphocytes  (Fig.  49,  1;  are  a  form,  of  blood  corpuscle  consisting 
of  a  round  nucleus  containing  block-like  masses  of  chromatin,  and  of 
a  narrow  rim  of  protoplasm.  Plasma  cells  (Fig.  49,  p)  are  derived  from 
lymphocytes  by  an  increase  in  their  protoplasm  which  stains  deeply  with 
most  stains,  but  especially  with  basic  dyes  such  as  methylene  blue.  It 
is  a  dense  protoplasm  which  contains  no  distinct  coarse  granules.  A  clear 
area  around  a  diplosome  or  a  group  of  centrosome  granules  may  be  found 
in  favorable  specimens.  The  resting  wandering  cells  (Fig.  49,  r.  w.j  are 
said  also  to  be  derived  from  lymphocytes.  They  resemble  connective 
tissue  cells  (fibroblasts;  but  do  not  produce  fibers.  Their  nuclei  are  smaller, 
darker,  and  more  irregular.  Their  protoplasm,  which  extends  in  irregular 
processes,  contains  scattered  coarse  granules  staining  deeply  with  basic 
stains.  These  cells  have  been  called  dasmatocytes.  In  amphibia  there 
are  connective  tissue  cells  with  slender 
processes  fuU  of  granules.  These  are 
described  as  producing  detached  frag- 
ments, and  so  were  named  dasmato- 
cytes. In  mammals  the  fragmentation 
has  not  been  obserA'ed  and  the  "das- 
matocytes" are  so  different  from  those 
of  amphibia  that  the  term  is  scarcely 
applicable.  The  resting  wandering 
cells  or  dasmatocytes  have  been  con- 
sidered varieties  of  mast  cells.  The 
mast  cells  (Fig.  49,  m)  are  characterized 
by  coarse  protoplasmic  granules  stain- 
ing intensely  with  basic  stains.  These 
granules  are  soluble  in  water  and  are 
poorly  preser\^ed  in  ordinary  sections.  The  nuclei  are  usually  round. 
Eosinophiles  (Fig.  49,  e;  also  have  coarse  granules,  but  they  do  not  stain 
with  basic  dyes;  they  have  great  affinity  for  acid  stains,  particularly  eosine. 
Their  nuclei  are  round  or  indented. 

The  free  cells  of  connective  tissue  occur  especially  along  the  courses 
of  small  blood  vessels.  They  will  be  better  understood  by  the  student 
after  examining  blood,  for  they  are  closely  related  to  the  white  corpuscles 
to  be  described  later.  All  forms  of  blood  corpuscles  are  to  be  found  at 
times  in  the  meshes  of  connective  tissue. 

The  intercellular  spaces  of  connective  tissue  are  of  special  importance. 
Between  the  fibril  bundles,  the  cells  and  the  elastic  network,  there  remain 
spaces  filled  with  fluid.  They  are  extensive  in  reticular,  mucous,  and 
loose  connective  tissue,  but  are  reduced  to  slender  channels  in  the  dense 


Fig.  49. — The  Cells  of  Loose  Connective 
Tissue,  the  Lowest  Row  from  a  Rabbit, 
the  Rest  from  .\  Guinea  Pig.  (After 
Maximow.) 

e.,  Eosinophile;  f.,  fibroblast;  I.,  lymphocyte; 
m.,  mast  cell;  p.,  plasma  cell ;  p.  w.,  resting 
wanderinsf  cell. 


48 


HISTOLOGY. 


forms.  Fluids  circulate  in  them,  conveying  nutriment  from  the  vessels 
to  epithelial  and  other  cells  and  conducting  waste  products  back  to  the 
vessels.  WTiite  blood  corpuscles  pass  out  between  the  endothehal  cells 
of  the  vessels  to  enter  these  spaces  in  which  they  may  travel  about  or 
multiply.  Some  corpuscles  may  originate  in  them,  formed  from  adjacent 
connective  tissue  cells.  The  intercellular  or  tissue  spaces  (lymph  spaces) 
differ  from  small  vessels,  either  blood  or  lymphatic,  in  having  no  endothelial 
walls;  and  the  tissue  fluid  which  they  contain  ordinarily  differs  from  either 
the  blood  plasma  or  the  lymph.  It  undoubtedly  resembles  lymph  with 
which  it  has  been  considered  identical. 

Summary  0}  connective  tissue.  Connective  tissue  consists  of  inter- 
cellular spaces  and  fluid,  white  fibers,  elastic  fibers,  and  cells.  It  sur- 
rounds the  various 
organs,  and  through 
it  pass  the  nerves, 
blood  and  lymphat- 
icvessels.  Its  spaces 
are  intermediate 
paths  between  the 
vessels  and  the  cells 
of  the  organs.  Its 
elastic  fibers  which 
though  varying  in 
size  are  not  divisible 
into  smaller  ele- 
ments, form  slender 
networks  or  coarse 
fenestrated  mem  - 
branes,  and  are  of 
exoplasmic  origin. 
Its  white  fibers  are  bundles  of  fibrils  cemented  together,  and  either  densely 
packed  or  loose  and  areolar.  Its  cells  are  those  which  produce  the  fibers, 
together  with  fat  and  pigment  cells,  and  various  forms  lodged  in  the 
intercellular  spaces.  These  include  lymphocytes,  plasma  cells,  resting 
wandering  cells,  mast  cells,  and  eosinophiles. 

Tendon. 

Tendons  consist  essentially  of  very  dense  connective  tissue  with 
parallel  fibers.  The  dense  tissue  as  seen  in  cross  section.  Fig.  50,  is  covered 
by  a  sheath  of  ordinary  connective  tissue,  prolongations  of  which  extend 
into  the  substance  of  the  tendon.     There  thev  unite  to  form  a  network 


Seplum.        Blood  vessel. 


Tendon  bnndle. 


Fibrous  sheath. 


Fig.  50.— Fro.m  .\  Cross  Section  of  a  Tendon  from  an  Adult  M.'^n. 
X40. 


TENDON. 


49 


-P- 
-tc. 


of  partitions  or  septa.  This  ordinary  connective  tissue  contains  nerves 
which  supply  the  tendon,  to  be  further  described  on  page  103 ;  also  blood 
vessels  in  relatively  small  number,  and  lymphatic  vessels  which  are 
confined  to  the  outer  sheath.  The  septa  surround  bundles  or  fasciculi 
of  tendon  fibers,  called  "secondary  tendon  bun- 
dles" in  distinction  from  the  smaller  "primary 
bundles"  of  which  they  are  composed.  The  latter 
are  groups  of  fibers  more  or  less  definitely  sur- 
rounded by  wing- like  processes  of  the  tendon  cells, 
which  appear  as  dots  in  Fig.  50,  but  are  clearly 
shown  in  Fig.  51.  The  tendon  cells  are  charac- 
terized by  their  compressed  branches  which  extend 
between  and  around  the  fiber  bundles,  anastomos- 
ing with  similar  branches  of  neighboring  cells. 
The  fibers  are  white,  consisting  of  collagen  (the 
gelatin-producing  substance)  and  of  tendo-mucoid 
which  may  be  found  in  the  cementing  matrix. 
Elastic  elements  are  said  to  occur  in  small  quan- 
tity especially  near  the  cells  and  their  processes. 
Intercellular  spaces  are  very  small  and  are  not 
shown  in  the  figure.  In  longitudinal  sections,  Fig. 
52,  the  parallel  arrangement  of  the  fibers  is  ap- 
parent, and  the  nuclei  are  in  rows.  The  protoplasm  is  often  indistinguish- 
able, but  in  special  preparations  from  delicate  tendons  it  appears  as  a 
thin  folded  layer  with  plate-like  projections.  Fig.  53. 


Fig.  51. — From    the    Calca- 
NEAN     Tendon     [Tendo 
AcHiLLis]  OF  A  Rabbit. 
(After  Prenant.) 

p.  b.,  Primary  bundle;  sh., 
sheath  of  the  bundle;  p., 
process  from  a  tendon  cell, 
t.  C,  extending  into  a  pri- 
mary bundle.  The  entire 
figure  is  a  portion  of  a  sec- 
ondary bundle. 


i-^^&Grps^^-:-^ 


^L-5£S3^;^- 


FiG.  52.— Longitudinal  Section  of  a  Calca- 
NEAN  Tendon  of  Man. 


Fig.  =;3.— Tendon  Cells  from  the  Tail  of 
a'Rat.  Stained  with  Methylene  Blue, 
Intra  \'itam.    (Huber.) 


The  fibrous  sheath,  vagina  fibrosa,  which  surrounds  the  tendon,  may 
contain  a  cavity  filled  with  fluid.  Such  a  tendon  sheath  is  called  a  mucous 
sheath,  vagina  mucosa.  The  cavity  arises  as  a  cleft  in  the  embryonic 
connective  tissue  and  its  walls  are  formed  of  mesenchjTnal  epithehum. 
The  cells  have  become  flattened  and  the  fibers  felted  together  to  bound 
4 


5° 


HISTOLOGY. 


the  space.  It  contains  a  fluid  like  that  of  the  joint  cavities,  being  chiefly 
water  and  a  mucoid  substance  (not  a  true  mucin)  which  renders  it  viscid, 
together  with  proteid  and  sahs.  The  function  of  the  mucous  sheath  is 
to  facihtate  the  movements  of  the  tendon.  By  its  formation  the  tendon 
is  freed  from  the  local  connection  with  surrounding  tissue,  and  the  sheath 
generally  occurs  Avhere  such  connection  would  especially  interfere  with 
motion.  The  mucous  hursae  are  similar  structures  in  relation  with  muscles 
or  bones.  The  joint  cavities,  to  be  described  later,  belong  in  the  same  class, 
having  a  similar  origin  and  function. 

Aponeuroses  and  fasciae  are  connective  tissue  formations,  resembling 
tendon  in  possessing  a  more  or  less  regular  arrangement  of  cells  and 
fibers.     Elastic  elements  may  be  abundant. 


m 


Cartilage. 

Cartilage  is  a  derivative  of  mesenchyma  which  may  develop  as  shown 
Fig.   54,  A.     The  mesenchymal  cells  multiply  and  become  crowded 

tosrether  so  that  the  inter- 


Mes 


-cC— 


Cart. 


Q 


Fig.  54. — Diagrams  of  the  Dkvelopment  of  Cartilage 

FROM    MESENCH'SMA. 


cellular  spaces  are  obliter- 
ated. Thus  precartilage  is 
formed,  consisting  of  large 
closely  adjacent  cells  sepa- 
rated from  one  another  by 
thin  walls  staining  red  w^ith 
eosin.  Precartilage  becomes 
cartilage  by  the  thickening 
of  these  exoplasmic  walls 
which  become  changed 
chemically  so  that  they  stain 

A,  Based   upon   Studnickas   studies  of  fish;  B.  upon  Mall's         b  lu  C     with     hacmatOXylin. 
studv  of  mammals.     Mes.,  Mesenchvma ;    Pre.  Cart.,  pre-         .^      >-«  v.-  j 

cartilage;  Cart.,  cartilage.  '  Tlic  cndoplasm  may  shrink 

from  them  so  that  the  cell  is  seen  to  occupy  a  little  cavity  in  the 
exoplasmic  matrix.  The  cavity  is  a  lacuna  and  if  the  matrix  around  it 
appears  to  form  a  special  wall  for  the  lacuna,  the  wall  is  called  a  capsule. 
The  cell  is  the  center  of  matrix  formation,  producing  it  in  concentric 
layers;  and  the  capsule,  being  that  portion  of  the  matrix  nearest  the  cell, 
is  the  part  most  recently  formed.  The  cells  consist  of  a  spongy  protoplasm 
due  to  vacuoles  of  fat,  and  to  spaces  from  which  glycogen  has  been  removed. 
Within  a  lacuna  the  cells  may  divide  by  mitosis  so  that  there  may  be  four 
or  eight  in  one  capsule.  Ordinarily  they  move  apart,  by  resorbing  the 
adjacent  matrix  fStohr)  or  by  forming  new  ground  substance  w^hich  forces 
them  apart  (Mall).     New  exoplasmic  walls  develop  between  them,  pro- 


CARTILAGE.  5 1 

ducing  characteristic  groups  and  rows  of  cells  such  as  are  shown  in  the 
diagram.  It  has  been  reasserted  that  some  of  the  cells  undergo  a  mucoid 
degeneration  and  become  lost  in  the  matrix.  Around  the  entire  cartilage 
of  the  adult  there  is  a  connective  tissue  envelope,  the  perichondrium, 
containing  undifferentiated  cells  which  by  growth  and  division  become 
cartilage  cells.  They  are  added  to  its  surface.  The  young  generations  of 
cartilage  cells  are  therefore  at  the  periphery,  and  the  old  are  in  the  center 
of  the  cartilage.  Between  them  an  interesting  series  of  cytomorphic 
changes  may  be  seen.  The  perichondrium  contains  vessels  and  nerves. 
Blood  vessels  may  extend  into  the  cartilage  of  young  embryos,  and  into 
cartilages  which  are  being  replaced  by  bone,  but  ordinarily  cartilage  is 
non-vascular,  receiving  its  nutriment  by  diffusion  through  the  matrix. 
In  surgical  operations  the  preservation  of  the  perichondrium  may  be  of 
importance,  since  it  can  produce  new  cartilage. 

Fig.  54,  B,  presents  Prof.  Mall's  idea  of  the  formation  of  precartilage 
in  mammals,  differing  from  that  just  described  which  followed  Dr.  Stud- 
nicka's  work  on  fishes.  In  B,  by  the  development  of  fibrils  which  are 
exoplasmic  structures  staining  with  eosin,  the  nuclei  and  endoplasm 
become  "extruded  from  the  syncytium"  and  lie  in  the  intercellular  spaces. 
The  exoplasm  becomes  transformed  into  the  matrix  of  the  cartilage. 
The  crowded  condition  of  the  nuclei  in  precartilage  makes  it  difficult  of 
interpretation. 

Glycogen,  which  occurs  in  cartilage  cells,  is  a  carbohydrate  resembling 
starch  and  known  as  "animal  starch."  It  is  soluble  in  water,  and  soon 
after  death  is  converted  into  glucose.  For  these  reasons  it  disappears 
from  ordinary  sections.  Fresh  tissues  preserved  in  strong  alcohol,  and 
stained  with  tincture  of  iodine,  exhibit  glycogen  as  brownish  red  masses, 
tending  to  be  round,  but  often  not  sharply  outlined.  Glycogen  is  abundant 
in  embryos  in  the  epithelium  of  the  skin,  in  liver  cells  and  striated  muscles 
and  in  cartilage  cells.  It  is  found  in  similar  situations  in  the  adult,  espe- 
cially in  well-nourished  individuals,  but  is  apparently  not  as  abundant 
relatively  as  in  the  embryo.  It  occurs  also  in  other  cells.  Its  production, 
like  that  of  fat,  may  be  considered  a  nutritive  rather  than  a  glandular 
phenomenon. 

The  matrix  of  cartilage  chemically  is  a  mixture  of  collagen,  chondro- 
mucoid,  chondroitin  sulphuric  acid  (in  combination),  and  albuminoid  sub- 
stances (albumoid).  [The  old  term  "chondrin"  really  means  little  else 
than  the  matrix  of  cartilage.]  The  collagen  may  occur  in  white  fibers 
which  abound  in  the  matrix  of  that  form  of  cartilage  called  fibro-cartilage. 
Elastic  fibers  predominate  in  the  matrix  of  elastic  cartilage.  If,  however, 
on  ordinary  microscopic  examination  the  matrix  appears  homogeneous, 


52 


HISTOLOGY. 


it    denotes    a    hyaline   cartilage.    Hyaline,    elastic,    and    fibro-cartilages 
require  special  examination. 

Hyaline  cartilage  macroscopically  is  a  pale  bluish  or  pearly  trans- 
lucent substance,  firm  and  elastic.  It  forms  some  of  the  cartilages  of  the 
larynx,  and  those  of  the  trachea  and  bronchi,  the  nose,  ribs  and  generally 


pi     «(  ■,> 


p^w^ 


.®^<^^ 


(^ 

..'*» 


ABC 

Fig.  55.— The  Three  Types  of  Cartilage:  A,  Hyaline;  B,  Elastic;  C,  Fibrous.    (Radasch.) 
a,  b,  Outer  and  inner  layers  of  perichondrium  ;  c,  young  cartilage  cells  ;  d,  older  cartilage  cells  ;  e,  f,  cap- 
sule ;  g,  lacuna. 

the  covering  of  the  joint  surfaces,  together  with  the  cartilaginous  skeleton 
of  the  embyro.  Its  matrix,  though  apparently  homogeneous,  Fig.  55,  A, 
is  actually  fibrillar,  as  shown  by  its  behavior  under  polarized  light,  and  by 
its  separation  into  fibers  after  artificial  digestion.  Whether  its  lacunae 
are  connected  with  each  other  by  small  canals  as  in  bone  and  in  the  carti- 
lage of  some  invertebrates,  is  very  doubtful.  Such  canals  as  have  been 
observed  are  ascribed  to  shrinkage  caused  by  reagents.  Sometimes,  as 
in  portions  of  the  laryngeal  and  costal  cartilages,  the  matrix  may  develop 
coarse  fibers,  neither  white  nor  elastic,  which  have  a  luster  hke  asbestos. 
In  old  age,  and  even  by  the  twentieth  year  in  the  case  of  some  laryngeal 
cartilages,  lime  salts  may  be  deposited  in  the  matrix,  first  as  granules 
but  later  combining  to  form  shells  enclosing  the  cartilage  cells.  Calcified 
cartilage,  together  with  calcified  tendon  and  other  structures,  should  not, 
however,  be  regarded  as  bone. 

Elastic  cartilage  is  a  pale  yellowish  structure  containing  in  its  matrix 


CARTILAGE,  53 

granules,  fibers,  or  networks  of  elastic  material.  Fig.  55,  B,  and  Fig.  56. 
Specific  elastic  tissue  stains  are  as  applicable  to  cartilage  as  to  connective 
tissue,  and  should  be  used  in  all  cases  of  doubt  as  to  the  nature  of  the  fibers. 
The  elastic  elements  are  found  near  the  cells,  but  agreement  has  not  been 
reached  as  to  whether  they  arise  in  the  matrix  or  in  the  exoplasm.     Elastic 


1 


K 


^ — 


1.  2.  3. 

Fig.  56. — Elastic  Cartilage.    X  240. 
I,  Portion  of  a  section  of  the  vocal  process  of  an  arytaenoid  cartilage  of  a  woman  thirty  years  old;  the 
elastic  substance  is  in  the  form  of  granules.     2  and  3.  Portions  of  sections  of  the  epiglottis  of  a  woman 
sixty  years  old  ;  a  fine  network  of  elastic  fibers  in  2,  a  denser  network  in  3.     z,  Cartilage-cell,  nucleus 
invisible;  k,  capsule  (?). 

cartilage  occurs  in  the  external  ear  and  the  auditory  (Eustachian)  tube;  also 
in  the  epiglottis,  the  cuneiform  and  comiculate  cartilages,  and  the  vocal 
process  of  the  arytaenoid  cartilages,  the  last  group  being  parts  of  the  larynx. 
Fibrocartilage,  Fig.  55,  C,  appears  as  a  cartilaginous  modification 
of  dense  connective  tissue.  A  chondro-mucoid  matrix  forms  among  the 
fibers,  and  the  cells  which  occur  singly  or  i 

in  small  groups  at  considerable  intervals,  ^'  ''        J 

are  surrounded  by  capsules.     Fibrocar-  '*''  ^ 

tilage  is  found  in  the  inter\^ertebral  liga-  j 

ments,  Fig.  57,  in  the  symphysis  pubis,     g—rr-^  ' 

around  the  mandibular  and  sternoclavicu-  j ..  /    ,, 

lar  ioints,  at  the  head  of  the  ulna,  in  the        /  'j^.^    /  /''"'; 

ligamentum  teres  of  the  hip-joint  and  in     ^     f"®  ;  ,     ^^ 

other  places  associated  with  joints.      Ves-     ^      "■'-"  /  ^t   li/'f  /  - 

icular  supporting  tissue  is  the  nsLmt  given  [:•■,•-,:  •>    > 

to  a  tissue  found  in  lower  animals,  resem-      fig.  57.— from  a  horizontal  section 

.      .  of  the  Intervertebr.al  Disk  of  M.an. 

bling    precartilage,  and  COnSlStmg   of    ves-  g,  Fibrillar  connective  tissue;  z.canilage- 

.             ,.          .  -     _                  .            J          11          o       1  cell     (nucleus    invisible) ;     k,   capsule 

ICular  cells  with  firm  resistant  wails,     ouch  surrounded    by    calcareous    granules. 

cells  may  occur  singly.     They  have  been 

described  in  various  tendons,  and  in  the  sesamoid  bone  in  the  tendon  of 

the  human  peroneus  longus. 

BOXE. 

Bone  develops  relatively  late  in  embryonic  life,  after  the  muscles, 
nerves,  vessels,  and  many  of  the  organs  have  been  formed.     x\t  this  time 


54 


HISTOLOGY. 


the  skeleton  consists  of  hyaline  cartilages  which  correspond  with  the  bones 
of  the  adult.  Around  the  cartilages,  or  in  some  places  quite  apart  from 
them,  the  bone  is  formed  in  the  following  manner: 


Calcifying  connective 
tissue  bundles. 


Bone  matrix 


Osteoblasts 


Fig.  58.— From  a  Section  of  the  Mandible  of  a  Hu.man  Fetus  Foi'r  Months  Old.     X  240. 

In  the  embryonic  connective  tissue  certain  homogeneous  strands 
become  apparent,  staining  deeply  with  eosin,  Fig.  58.  These  represent 
the  matrix  or  ground  substance  of  bone,  and  are  considered  either  trans- 
formations of  the  exoplasm  of  the  neighboring  cells,  or  as  secretions  of  those 

cells,  or  as  modifications 
of  connective  tissue  fibrils. 
They  blend  with  the  con- 
nective tissue  as  shown  in 
the  lower  part  of  the  figure. 
As  these  strands  become 
distinct,  they  are  seen  to  be 
covered  with  peculiar  cells  of 
mesenchymal  origin  which 
tend  to  form  a  distinct 
layer.  Since  they  produce 
bone  they  are  called  osteo- 
blasts. {Blast  is  a  designa- 
tion for  a  formative  cell,  and 
is  used  in  many  combina- 
tions with  a  prefix  denoting 
the  structure  which  it  pro- 
duces.) Osteoblasts  are 
sho^\^l  in  Figs.  58  and  59.  They  are  cells  with  rounded  nuclei  and  abun- 
dant protoplasm,  varying  in  shape  from  flat  to  columnar,  often  being 


*i. 


Fig.  59.— Part  of  a  Cross  Section  of  the  Shaft  of  the 
Hl-.merls  of  a  Hu.man  E.mbryo  Four  Months  Old. 
.  X560. 


BONE. 


55 


triangular  and  resting  against  the  strand  of  bone  either  by  their  base  or 
apex.  They  form  bone  only  along  that  surface  which  is  applied  to  the 
matrix.  As  the  strand  of  bone  grows  broader  through  their  activity,  it 
encloses  here  and  there  an  osteoblast  which  becomes  thereby  a  hone  cell 
(Fig.  59).  Apparently  bone  cells  do  not  divide,  and  if  they  produce  matrix, 
thus  becoming  more  widely  separated  from  each  other,  it  is  only  to  a  slight 
extent  and  in  young  bones;  they  are  therefore  quite  inactive.  Each  bone 
cell  occupies  a  space  in  the  hyaline  matrix,  called  as  in  cartilage,  a  lacuna, 


Osteoblasts. 

Haversian  canals  in 
the  process  of  forma- 
tion. 


Blood  vessels. 
Perichondral  bone. 


Finished      Haversian 
canal. 


Empty  lacunae. 


Osteoclast. 


Endochondral      border- 
line. 


Endochondral  bone. 


Fig.  60. — Portion  of  a  Cross  Section  of  a  Tubular  Bone  of  a  Newborn  Kitten. 

but  unlike  the  lacunae  of  cartilage  those  in  bone  are  connected  by  numerous 
delicate  canals,  the  canaliculi.  In  ordinary  specimens  the  canaliculi  are 
visible  only  as  they  enter  the  lacunae,  which  are  thus  made  to  appear  stellate. 
The  matrix  around  the  lacunae  resists  such  acids  as  destroy  the  ordinary 
matrix  and  thus  may  be  isolated  in  the  form  of  "bone  corpuscles."  The 
"corpuscles"  correspond  with  the  capsules  of  cartilage.  The  bone  cells 
nearly  fill  the  lacunae  and  send  out  slender  processes  into  the  canaliculi. 
These  may  anastomose  with  the  processes  of  neighboring  cells,  as  can 
be  seen  in  the  embryo,  but  it  is  considered  doubtful  if  this  condition  is 


56 


HISTOLOGY. 


retained  in  the  adult.     The   processes,  moreover,  are  so  fine  as  to  be 
invisible  ordinarily,  and  formerly  their  existence  was  denied. 

The  strands  of  bone  containing  bone  cells,  and  beset  with  osteoblasts, 
increase  in  size  and  unite  so  as  to  enclose  areas  of  embryonic  connective 
tissue  containing  blood  vessels,  as  shown  in  the  upper  part  of  Fig.  60, 
and  in  the  diagram.  Fig.  61.  The  connective  tissue  surrounding  the 
entire  network  of  bone  becomes  differentiated  into  a  distinct  layer,  the 
periosteum.  This  includes  an  outer  stratum  of  ordinary  connective  tissue 
(not  drawn  in  the  figures),  a  middle  layer  of  dense  fibrous  tissue,  and  an 
inner  cellular  layer  including  the  osteoblasts  in  contact  with  the  outer 
surface  of  the  bone.  Fig.  61  shows  the  way  in  which  a  portion  of  this 
inner  stratum  may  be  enclosed  in  the  bone  matrix.     It  is  about  to  occur 

around  the  blood  vessel, 
B.  v.,  and  has  taken  place 
in  the  space  H.  C^  Within 
such  an  enclosure  the  osteo- 
blasts continue  to  form 
bone  in  concentric  layers  or 
lamellae,  thus  gradually  re- 
ducing the  central  space 
until  it  contains  only  a  few 
cells  and  the  blood  vessels 
as  in  H.  C^.  Such  spaces 
occur  abundantly  in  adult 
bone,  and  are  called  Haver- 
sian canals  (in  recognition 
of  the  anatomist  Havers). 
They  are  always  surrounded 
by  concentric  lamellae,  or 
layers  of  bone,  of  which  the  innermost  is  the  youngest.  Between  these 
Haversian  systems  there  are  irregular  lamellae,  called  interstitial  lamellae, 
and  sometimes  a  blood  vessel  nms  through  them,  not  surrounded  by  con- 
centric layers.  It  is  said  to  occupy  a  Volkmann^s  canal.  Transitions  from 
a  Volkmann's  to  an  Haversian  canal  are  gradual,  and  are  made  not  by  a 
change  in  the  canal  but  by  a  rearrangement  of  the  surrounding  lamellae. 
Coarse  fibers  may  extend  from  the  periosteum  into  the  interstitial  lamellae, 
known  as  Sharpey^s  fibers.  They  consist  of  more  or  less  calcified  bundles 
of  connective  tissue  fibers,  including  both  white  and  elastic  elements,  though 
chiefly  the  former.  If  abundant,  the  periosteum  is  most  closely  adherent 
to  the  bone.  They  are  absent  from  the  Haversian  systems.  Besides  the 
interstitial  and  concentric  lamellae,  another  set  is  deposited  under  the  peri- 


FiG.  61. — Diagram  of  the  Development  of  Bone. 
(In  part,  after  Duval.) 
f .,  Fibrous  layer  of  periosteum  ;  o.,  osteogenic  layer  of  perios- 
teum ;  OS.,  osteoblast;  b.  C,  bone  cell;  B.  V.,  blood  ves- 
sel ;  H.  C,  beginning  Haversian  canal  ;  H.  C-.,  complete 
Haversian  canal;  i.  I.,  interstitial  lamellae;  c.  I., concen- 
tric lamellae  ;  Sh.,  Sharpey's  fibers. 


BONE. 


57 


Periosteum. 

Periosteal 

lamellae. 

Haversian 

canals. 


osteum,  parallel  with  the  surface  of  the  bone,  the  periosteal  lamellae  [outer 
circumferential  or  outer  ground  lamellee].  If  the  bone  is  hollow,  having  a 
marrow  cavity,  similar  lamellae  may  be  deposited  over  the  inner  surface  of 
the  shaft  by  a  formative  layer  called  the  endosteum.  These  lamellae  are  end- 
osteal lamellae  [inner  ground  or  circumferential  lamellae,  marrow  lamellae] 
and  they  line  the  marrow  cavity.  The  four  sets  of  lamellae  are  sho-wm  in 
Fig.  62. 

Lamellar  bone  is  compact,  differing  notably  from  the  spongy  network 
of  trabeculae  seen  in  the  embryo.  Compact  bone  is  found  in  the  outer  parts 
of  the  long  and  fiat  bones  and  as  a  thin  outer  layer  in  short  bones.  Spongy 
bone  is  found  in  the  interior  of  long  bones,  and  of  fiat  bones  (where  it  is 
called  diploe),  and  it  constitutes  the  greater  part  of  short  bones  and  epi- 
physes. It  is  due  in  part  to  the  persistence  of  the  embryonic  trabeculae, 
and  in  part  to  the  reduction 


of  compact  bone  to  slender 
spicules  through  processes 
of  absorption.  Scarcely  has 
bone  formed  before  portions 
of  it  begin  to  be  resorbed. 
The  osteoblasts  disappear 
locally  and  in  place  of  them 
there  are  large  irregular 
masses  of  protoplasm  con- 
taining several  separate  nu 
clei.  .  Tlie  idea  that  these 
structures  arise  by  the  fusion 
of  several  osteoblasts  is  not 
accepted;  the  nuclei  are  thought  to  arise  by  repeated  division,  mthin  a 
mass  of  protoplasm  which  enlarges  but. does  not  divide.  The  form  of 
giaSTcelFresulting  is  called  an  osteoclast,  from  its  supposed  function  of 
destroying  bone.  The  osteoclasts.  Fig.  60,  are  often  seen  in  hollows 
which  they  are  thought  to  have  excavated  in  the  ground  substance,  and 
which  are  called  Wowshifs  lacunae.  There  seems  to  be  no  satisfactory 
evidence  that  the  osteoclasts  are  the  cause  rather  than  a  product  of  those 
conditions  which  lead  to  the  dissolution  of  bone.  The^prqcess  of.  reso;:^- 
tion  is  of  the  greatest  importance,  since  it  prevents  bones  from  becoming,, 
solid  and  heavy.  While  new  bone  is  forming  on  the  periosteal  surface, 
old  bone  is  being  dissolved,  both  around  the  marrow  cavity  and  in  the 
deeper  Haversian  canals.  This  process  produces  most  of  the  spongy  bone 
of  the  adult, 

Reviewmg  the  preceding  paragraphs,  it  may  be  said  that  bone  appears 
first  as  strands  of  ground  substance  produced  by  osteoblasts  derived  from 


Marrow. 


Fig.  62. — From  a  Cross  Section  of  a  Metacarpal  of 

Man.     X  5°- 

Resorption  line  at  h. 


58 


HISTOLOGY. 


r^; 


mesenchyma.  The  osteoblasts  may  be  enclosed  by  the  matrix  which  they 
form,  thus  becoming  bone  cells.  The  trabeculae  of  bone  produced  in 
this  manner  unite  in  a  network,  described  as  spongy  bone.  By  the  deposi- 
tion of  new  layers  or  lamellae  of  bone,  which  conform  with  the  surfaces 
on  which  they  are  laid  down,  the  spongy  bone  becomes  compact.  By 
resorption  of  the  inner  part,  the  marrow  cavity  forms  and  parts  of  the 
compact  bone  become  spongy.  It  remains  to  consider  the  substances 
and  appearances  of  adult  bone,  and  to  describe  the  manner  in  which  the 
cartilages  are  replaced  by  bone. 

The  matrix  of  bone  is  at  first  uncalcified  and  soft,  apparently  homo- 
geneous, but  actually  con- 
sisting of  cemented  fibrils. 
It  consists  chiefly  of  col- 
lagen— the  gelatin-pro- 
ducing substance,  and  of 
a  mucoid  called  osseo- 
mucoid. Through  it  there 
may  be  distributed  fine 
elastic  fibers  (said  to  be 
lacking  in  the  bones  of 
the  vertex  of  the  skull) 
besides  the  coarser  con- 
nective tissue  bundles  of 
Sharpey.  Soon  after  this 
ib- 


organic  matrix  isesl  _ 
lished,  calcification  begins 
by  the  deposition  of  lime 
salts  either  in  or  between 

Fig.  63. — From  a  Lum.iu  ijinal  Section  of  a  Human  .-,        r^     •^  /-\  r,       n-f 

Metacarpal.    X  30.  the    fibnls.       OvCr   80    % 

Fat  drops  are  seen  in  the  Haversian  canals.    At  x  Haversian  canals 
Ofien  on  the  outer,  and  at  xx  on  the  inner  surface  of  the  bone. 


Haversian 
canals. 


Ground 
substance. 


Periosteum. 


Fat  drops. 


of  the  inorganic  matter 
is  calcium  phosphate, 
Cdi.^{VO^^_,  the  remainder  including  chlorides,  carbonates,  fluorides  and 
sulphates  of  calcium,  sodium,  potassium,  and  magnesium.  The  proper- 
ties of  bone  depend  largely  upon  the  intimate  blending  of  the  organic 
and  inorganic  constituents,  possibly  in  chemical  combination.  The  two 
parts  may  be  separated,  however.  Acids  remove  the  salts  leaving  the 
organic  portion  as  a  flexible  counterpart  of  the  entire  bone.  Heat  or 
maceration  may  be  employed  to  destroy  the  organic  part.  Microscopic 
preparations  are  made  in  either  way,  but  usually  from  decalcified  bones. 
All  of  the  drawings  thus  far  referred  to  were  of  such  specimens. 

The  cross  section  of  a  decalcified  long  bone  of  an  adult,  Fig.  62, 


BONE. 


59 


shows  the  periosteum  on  its  outer  surface.     Iji_Ja^oraJ2k»sp.ecinierisJiJa-, 
seen  to  include  an  outer  vascular,  rather  loose  connec.tiyx  tissue  layer^, 
andean  inner  dense  fibro-elastic  layer,  in  which  elastic  elements  predom.:;;^ 
inate.     Into  this  layer  the  tendons  are  inserted,  which  means  that  they 
blend  with  it  and  may  contribute  to  the  fibers  penetrating  the  bone.     The 
innermost  cellular  layer  of  the  periosteum  has  become  reduced  to  oc- 
casional osteoblasts.     These   may  multiply   after   an^mjury;    in  young 
individuals,  if  the  periosteum  is  sHt  andjLhe  sliaft  of  bone  shelled  out,  they^ 
may~proHucea  new  bon^    The  cross  section  further  shows  the  contents 
of  the  Haversian  canals,  which  include  one  or  two  blood  vessels,  and  a 
few  connective  tissue  or  fat  cells. 
Nerve  fibers  which  are  found  in 
the  periosteum,  where  they  some- 
times terminate  in  lamellar  cor- 
puscles (page  107),  have  been  de- 
scribed   as    extending_^  into_the 
Haversian  canals.-    They  are  not 
easily  detected  there.     Lamellae 
may   be   observed   as   indistinct 
layers.     They  are  said  to  be  due 
to  the  differences  in  direction  of 
the  fibrils    which   they    contain, 
as  shown  under  polarized  light. 
They  may  also  represent  differ- 
ences in  texture,  from  variations 
in  the  food  supply  at  the  time  of 
their   formation.      The    lacunae 
may  appear  either  in  or  between 
the   lamellae.     They  are  nearly 
filled  by  the  bone  cells,  which, 
however,   are   seldom  well   pre- 
served.    The  cells  are  generally  flattened,  parallel  with  the  lamellae,  and 
are  provided  with  processes  extending  into  the  canaliculi.     They  do  not 
fill  them  and  it  is  supposed  that  tissue  fluids  may  circulate  through  the 
lacunae  and  canaliculi.    Wandering  blood  cells  are  too  large  to  enter  them. 
The  lymphatic  vessels  are  limited  to  the_superfidalji)3^f^r  ^f  <-hp  ppri^^^tpnm. 
The  blood  supply  of  bone  is  abundant.     One  or  more  nutrient  arteries^ 
enter_a_,_bone  through  its  periosteum  and  break  into  branches  which  nm 
in  the  Haversian  canals,  thus  extending  tln-ough  to  the  m^yrQW  caaty- 
in5hich_,they  ramify  freely.     The  blood  vessels  and  Haversian  systems 
are  parallel  with  the  long  axis  of  the  bone,  so  that  they  are  cut  across  in 


•    '     V 

Fig.  64.— Cross  Section  of  Compact  Bone,  from 
THE  Shaft  of  the  Humerus,  showing  Three 
Haversian  Systems  and  Part  of  a  Fourth. 
(5Aar/^_)',fromBaile3''s"  Text-book  of  Histology.") 


6o 


HISTOLOGY. 


cross  sections.  In  longitudinal  sections  they  appear  as  in  Fig.  63.  Veins 
pass  back  from  the  marrow,  through  the  Haversian  canals,  emerging 
through  the  periosteum.  It  will  be  noticed  that  in  longitudinal  sections 
the  lamellar  systems  are  scarcely  distinguishable.  On  the  marrow  side,  the 
endosteum  forms  a  thin  fibrous  layer  containing  occasional  osteoblasts  and 
osteoclasts.     The  marrow  will  be  described  with  the  blood-forming  organs. 


Hyaline 
cartilage 


Osteogenic 

tissue. 


Center  of  cal 
cification. 


/ 


Perichondral    y 
bone. 


Fig.  65.— From  a  Dorso-p.alm.ar  Longitudinal  Section  of  a  Phalanx  of  the  Little 
Finger  of  a  Human  Fetus  Six  Months  Old.     X  60. 


Preparations  from  washed  and  dried  bones  show  only  the  calcareous 
framework.  Sections  made  by  sawing  show  macroscopically  an  arrange- 
ment of  the  spongy  bone  in  arches  and  trusses  to  resist  compression. 
Microscopic  sections  are  made  by  grinding  thin  sawed  slices. until  they 
become  translucent,  and  mounting  them  so  that  the  lacunae  and  canaliculi 
remain  full  of  air.  Since  the  air  is  refractive  it  appears  black.  Thus  the 
canaliculi  are  clearly  demonstrated,  as  in  Fig.  64.     They  extend  from  one 


BONE. 


6l 


lacuna  to  another,  connecting  the  different  Haversian  systems,  and  opening 
into  the  Haversian  canals. 


Enlarged 

cartilage 

cells. 


The  Relation  of  Bone  to  Cartilage. 

Some  bones  develop  quite  independently  of  cartilage.    These  include, 
besides  the  teeth,  the  so-called  membrane  hones  [iatramembranous,  connec- 
tive tissue  or  secondary  bones].    In  the  midst  of  the  embryonic  connective 
tissue,  spicules  of  bone 
are  formed  in  the  man- 
ner  already  described,  .* 
and  they  unite  to  form 
a  bone.   The  membrane 
bones  are  the  bones  of 
the  face,  and  the  fiat 
bones  of  the  skull; — the 
interparietal    or    upper 
part  of  the  occipital,  the 
squamous  and  tympanic 
parts  of  the  temporal, 
the  medial  pterygoid 
plate  of  the  sphenoid, 
the  parietal,  frontal, 
nasal,  lachrymal,  zygo- 
matic (malar),  and  pal- 
ate bones,  together  with 
the  vomer,  maxiUa,  and 
almost  the  entire  man- 
dible.    The  remaining 
bones,  being  preformed 
in  cartilage,  are  grouped 
as  cartilage  hones  [pri- 
mary bones].    They  de- 
velop   Hke    membrane 
bones  except  that  the  matrix  is  in  part  deposited  in  contact  with  cartilage 
in  the  following  maimer. 

Fig.  65  shows  a  longitudmal  section  of  a  developing  phalanx.  On_ 
either  side  of  the  sh_aft_a  strip  of  bone  is  seen,  formed  from  undifferentiated 
c_ells  of~~mesenchymal  origin,  situated  in  the  perichondrium^  It  is  caUed, 
perichondral  or  pmosteal  bone,  and  arises  Hke  membrane  bone,,^  As  a 
whole,"  it  forms  a  band  encircling  the  shaft  of  cartilage.  Within  it,  the 
cartilage  cells  have  enlarged,  and  divided  so  that  several  cells  may  be  in 


Perichon- 
dral bone. 


Periosteum. 


Fig.  66.— From 


^.  ^„     ^ DoRso-PALM.AR    Longitudinal    Section    of   a 

Middle-finger  Phalanx  of  a  Human  Fetus  Four  Months 
Old.    X  60. 


62 


HISTOLOGY. 


one  lacuna.  The  lacunae  also  have  increased  in  size.  The  matrix  in 
this  region  stains  a  deeper  blue  with  haematoxylin  than  elsewhere,  due 
to  the  deposition  of  lime  salts  within  it.     On  the  left  a  caA'ity  is  seen  ex- 


Hyaline  carti- 
lage (cells  in 
groups). 


Hyaline  car- 
li'lage  (cells 
enlarged). 


Periosteum. 


Osteoblasts 


Etidocliondral  bone 


Fig.  67.— Fro.m  a  Lo.vgituuinai.  Skction  of  the  Phalanx  ok  the  First  Finger  of  a  Human 
Fktus  of  Four  Month.s.     X  220. 

cavated  by  the  perichondral  tissue.  Several  such  buds  of  tissue  will  form, 
invading  the  cartilage  from  all  sides,  and  uniting  in  the  center  of  its  shaft. 
The  calcified  matrix  of  cartilage  dissolves  before  their  advance,  setting 


BONE. 


free  the  cartilage  cells  as  the  lacunae  are  broken  down.  This  has  occurred 
in  Fig.  66.  Th£-.tissue_which  enters  the  cartilage  is  a  vascular ^  emhryonir . 
connective  tissug,  containing  ostp.Qhlas,ts.^.ajid  forming  the^pjinnary  TnarroTy. 
Meanwhile  the  cartilage  has  continued  to  grow,  especially  in  length,  and 
the  cells  in  the  calcified  region  have  divided  so  as  to  form  rows.  The 
transverse  walls  of  the  lacunae  are  dissolved,  leaving  deep  blue  spicules 
of  calcified  matrix  extending  from  the  ends  of  the  cartilage  toward  its 
center.  Osteoblasts  arrange  themselves  on  these  spicules  and  form  bone, 
the  matrix  of  which  stains  red  with  eosin.     It  was  formerly  thought  that 

Haversian  Endochondral     Perichondral 

Periosteum.  depressions.  bone.  bone. 

>         /  .       /\ 


Haversian  canal. 


Calcified  matrix 
between  endo- 
chondral and 
perichondral 
bone. 


Blood  vessel. 


-"V  * 


>       V 


Remains  of  calci- 
fied matrix  of 
cartilag-e. 


Fig.  68.— From  a  Cross  Section  of  the  Shaft  of  the  Humerus,  from  a  Four  Months" 

HuM.^N  Fetus.     X  So. 

the  cartilage  cells  set  free  by  the  absorption  of  the  walls  of  the  lacunae 
became  osteoblasts,  but  now  they  are  considered  as  dying  cells  without 
further  function.  The  osteoblasts  belong  with  the  invading  cells.  As 
seen  in  Fig.  67,  both  the  perichondral  bone  on  the  surface  of  the  cartilage 
and  the  endochondral  bone  forming  within  it,  develop  like  membrane  bone. 
As  the  bone  grows,  the  older  parts  which  have  formed  around  the  calcified 
cartilage  are  resorbed,  and  in  the  shafts  of  adult  bones  probably 
no  trace  of  the  cartilage  remains.     In  the  ear  bones,  however,  calcified 


64 


HISTOLOGY. 


ep. 


cartilage  may  be  found  throughout  Hfe.  Fig.  68  sliows  a  part  of  the 
humerus  of  a  fetus  in  which  the  calcified  cartilage  remains,  forming  in  one 
place  a  boundary  between  endochondral  and  perichondral  bone.  The 
vascular  tissue  within  the  shaft  becomes  marrow, — a  reticular  tissue 
associated  with  fat  cells,  and  having  developing  blood  corpuscles  in  its 
meshes,  to  be  described  later. 

In  brief  review  it  may  be  said  that  cartilage  bones  are  formed  by  the 
deposition  of  perichondral  bone  on  the  outside  of  a  hyaline  cartilage, 
and  of  endochondral  bone  upon  the  lining  of  excavations  within  the  carti- 
lage. The  cartilage  is  nottransfornied  into  bone,  although  the  matrix 
in  part  becomes  calcified  and  encased  in  bone.  In  the  long  bones  this 
process  of  ossification  produces  a  shaft  of  bone  tipped  with  a  mass  of 

cartilage  at  either  end,  Fig.  69,  A,  B,  C. 
The  shaft  is  the  diaphysis;  the  cartilage 
ends  are  epiphyses.  At  various  times  after 
birth,  or  in  the  tibia  shortly  before  birth, 

IW\        I  j  osteogenic  tissue  invades  the  epiphysis  and 

p:  ]  "'"  '^P'— -;  I  gradually  replaces  its  cartilage  by  bone. 

Ill  A  layer  of  epiphyseal  cartilage  between 

the  epiphysis  and  diaphysis,  and  a  layer 
of  articular  cartilage  covering  the   joint 
surface  persist  longest.     Until  adult  life 
•^P--.   l\         theepiphysealj^artilage  grow 

ward  the  diaphysis,  and  the  addition  as  _ 
fast  as  it  forms  is  replaced  by  bone.  Thus^ 
the  epiphyseal  cartilage  is  an  essential 
provision  for  the  lengthwise  growth  of 
bones.  The  epiphyseal  cartilages  be- 
come entirely  calcified  at  different  ages  in  the  various  bones,  generally 
from  18  to  22  years,  at  which  time  the  epiphysis  is  said  to  unite  with  the 
diaphysis.  After  that  the  articular  cartilages  are  all  that  remain  of  the 
original  cartilaginous  structure  which  preceded  the  corresponding  bone. 

The  Joints. 

Bones  may  be  joined  in  two  ways,  either  by  a  synarthrosis  which 
allows  little  or  no  motion  between  them,  or  by  a  dlarlhrosis  which  permits 
them  to  move  freely  upon  one  another. 

In  a  synarthrosis  the  mesenchymal  tissue  between  the  adjacent  bones 
may  become  a  dense  connective  tissue,  either  like  a  fibrous  tendon  or  an 
elastic  ligament,  thus  forming  a  syndesmosis ;  or  it  may  become  cartilage, 
usually  of  the  fibrous  type,  making  a  synchondrosis.     The  sutures  are  forms 


art;-' 

Fig.    69. — Plan    of    Ossification    in    a 

Long  Bone,  Based  upon  the  Tibia. 
Cartilage   is  drawn   in   black,  and  bone  is 

stippled.     Art.,  Articular  cartilage;  ep., 

epiphysis;  diaph.,  diaphysis. 


JOIXTS. 


65 


s.f.- 


of  syndesmosis  in  which  the  serrate  borders  of  bones  are  connected  by 
short  fibrous  ligaments.  The  intervertebral  hgaments  are  synchondroses, 
each  consisting  of  a  fibrocartilage  which  has  at  its  center  a  soft  mucoid 
substance  containing  large  groups  of  cartilage  ....^;.^•^^:-fe. 

cells.     This  nucleus  pulposus  is  usually  inter-  .•/•••■•■.■:••>•.•.•:•.•. 

preted  as  the  remains  of  the  notochord,  but  • -:::-'-.V::^ 

V  car. ■:,:h':,:A: '.    ■  "■  .'■'..' :  ■•'■  ■;  :!^}, 

some  consider  that  the  notochord  is  entirely  WM.'^'r-'-:^--':'-':-.''-''WB- 

absorbed,  making  the  nucleus  pulposus  an 
independent  formation.  The  term  li'gament,  it 
will  be  noted,  is  applied  to  bands  of  various 
sorts,  fibrous,  elastic,  or  cartilaginous. 

In  a  diarthrosis  the  mesenchymal  tissue 
between  the  bones  remains  comparatively  loose 
in  texture  and  a  cleft  forms  in  it,  containing 
tissue  fluid.  This  is  the  joint  cavity,  Fig.  70. 
It  is  bounded  by  mesenchymal  cells  which 
spread  out  and  form  an  epithelium,  shown  in 
Fig.  71.  The  epithelium  may  fuse  with  the  articular  cartilage  so  that  the 
latter,  uncovered  by  perichondrium,  forms  a  part  of  the  wall  of  the  joint 


Fig.  70.— Phalangeal  Joint  from 

A  Four  Months'  Fetus. 
Car..  Cartilage;    j.  c,  joint  cavity; 

S.  f.,  stratum    fibrosum ;     s.   S., 

stratum  synoviale. 


■  ■  ® 

.©. 

.©. 

a 

© 

.®. 

"Q 

:,■© 

■  <3  ■.'■ 

'■)■ 

■<& " 

.©. 

& 

®  • 

a- 

■©'. 

-©■   . 

%■ 

©;■, 

S>   .  ■. 

0 

0 

Z  ■ 

.  s   ■. 

© 

-.Q 

^  <; 

^e> 

'.P 

Fig.  71. — An  Enlarged  Drawing  of  the  Left  Part  of  the  Joint  shown  in  Fig.  70. 
b.  v.,  Blood  vessel ;  car.,  cartilage ;  j.  c,  joint  cavity  ;  mes.  epi.,  mesenchymal  epithelium. 


cavity.  Articular  cartilages  are  usually  hyaline  layers  from  0.2  mm.  to 
5  mm.  thick,  becoming  thin  at  the  periphe^}^  The  cells  near  the  joint 
are  flattened  parallel  with  the  free  surface,  and  some  of  the  deeper  of  these 


66 


HISTOLOGY, 


are  said  to  have  lobed  nuclei.  The  flat  cells  are  succeeded  by  groups 
of  rounded  ones  which  are  described  as  having  protoplasmic  processes. 
In  the  deepest  layers  the  cells  tend  to  be  in  rows  perpendicular  to  the  joint 
surface  and  the  matrix  is  calcified.  In  Fig.  72  a  line  is  seen  separating 
the  calcified  from  the  uncalcified  part. 


^     ■& 


Hyaline 
cartilage. 


Calcified 
cartilage. 


^'1.   , 


Marrow 

(fat  cells). 
Blood  vessel. 


Fig.  72. — Vertical  Section  throi'gh  the  He.ad  of  a 
Metacarpal  of  an  Adult  Man.    x  50. 


r  ^-^SSu." 


Fig.  73.— Synovial  \'illi  with 
Blood  \'essels  from  a  Hu- 
^UN  Knee  Joint.     >;  50. 

The  epithelium  has  fallen  from  the 
apex  of  the  left  villus,  e.xposing 
the  connective  tissue. 


The  ioint  capsule  consists  of  an  outer  layer  of  dense  connective  tissue, 
the  stratum  fibrosum;  and  an  inner  loose  layer  of  which  the  mesenchymal 
epithelium  is  a  part,  the  stratum  synovial e  (Fig.  70).  The  fibrous  layer 
is  specially  thickened  in  various  places  to  form  the  ligaments  of  the  joint. 
It  may  cover  the  end  of  the  bone,  coming  between  it  and  the  joint  cavity; 
thus  the  distal  articular  surface  of  the  radius  is  covered  with  dense  fibrous 
tissue.  In  other  joints,  as  in  the  shoulder  and  hip,  such  tissue  forms  a  rim, 
deepening  the  socket  of  tlie  joint.  These  rims  are  called  labra  glcnoidalia. 
The  synovial  layer  consists  of  loose  tissue,  generally  with  abundant  elastic 
elements,  and  in  places  containing  fat  cells.  It  has  nerves  which  may 
terminate  in  lamellar  corpuscles,  numerous  blood  vessels,  and  lymphatic 
vessels  which  extend  close  to  the  epithelium.  The  epithelium  is  a  smooth, 
glossy  layer  of  connective  tissue  with  parallel  fibers  and  small  round  or 


TEETH. 


67 


stellate  cells  containing  large  nuclei.  They  may  be  spread  in  a  single  thin 
layer,  or  heaped  together,  making  an  epithelium  of  three  or  four  layers. 
The  synovial  membrane  may  be  thrown  into  coarse  folds  (plicae)  or  into 
slender  projections  often  microscopic  (villi).  The  synovial  villi.  Fig.  73, 
are  variously  shaped  but  are  usually  finger-like;  they  ordinarily  contain 
blood  vessels  and  impart  a  reddish  velvety  appearance  to  the  membrane. 
The  large  folds  of  embryonic  tissue  projecting  into  the  joint,  but  dlwsiys 
covered  with  the  mesenchymal  epithelium,  may  become  dense  fibrous 
articular  discs  such  as  are  interposed 
in  the  sternoclavicular  and  mandib- 
ular joints,  or  they  may  form  the 
fibrous  cartilage-like  menisci  of  the 
knee  joint.  Nerves  and  blood  ves- 
sels are  absent  from  the  discs,  men- 
isci, and  labra  glenoidalia. 

Synovia  [synovial  fluid]  is  94  % 
water,  the  remainder  being  salts, 
proteids,  and  mucoid  substances, 
together  with  fat  drops  and  frag- 
ments of  cells  shed  from  the  mem- 
brane. 


Crown. 


Dental 
cavitv. 


^:«i 


—  Neck. 


Cement. 


Teeth. 

A  tooth  consists  of  three  parts, 
crown,  neck,  and  root  or  roots.  The 
crown  is  that  portion  which  projects 
above  the  gums;  the  root  is  the  part 
inserted  into  the  alveolus  or  socket 
in  the  bone  of  the  jaw;  and  the  neck, 
which  is  covered  by  the  gums,  is  the 
connecting  portion  between  the  root 
and  cro^ATi.  A  tooth  contains  a  dental 
cavity  filled  with  pulp.  The  cavity 
is  prolonged  through  the  canal  of  the 
root  to  the  apex  of  the  root  where 

it  opens  to  the  exterior  of  the  tooth  at  the  foramen  apicis  dentis.  The 
foramen  is  shown,  but  is  not  labelled,  in  Fig.  74.  The  solid  portion  of 
the  tooth  consists  of  three  calcified  substances,  the  dentine  or  ivory  (sub- 
stantia eburnea),  the  enamel  (substantia  adamantina),  and  the  cement 
(substantia  ossea).  Of  these  the  dentine  is  the  most  abundant.  It  forms 
a  broad  laver  around  the  dental  cavity  and  root  canal,  and  is  interrupted 


Fig.  74. — Longitudinal  Ground  Section  of  a 
HU.M.A.N  Incisor  Tooth.    X  4- 


68 


HISTOLOGY. 


My 


,M 


DR. 


only  at  the  foramen.  Nowhere  does  the  dentine  reach  the  outer  surface 
of  the  tooth.  In  the  root  it  is  covered  by  the  cement  la3^er  which  increases 
in  thickness  from  the  neck  toward  the  apex;  and  in  the  crown  it  is  enclosed 
by  the  broad  layer  of  enamel.  The  enamel,  however,  becomes  thin  toward 
the  neck,  where  it  meets  and  is  sometimes  overlapped  by  the  cement.  The 
pulp,  dentine,  and  cement  are  of  mesenchymal  origin,  the  dentine  and  cement 
being  varieties  of  bone.  The  enamel  is  an  ectodermal  formation,  but  so 
intimately  associated  with  the  others  that  it  may  be  described  with  them. 

In  the  human  fetus  of  about  two  months  the  ectoderm  covering  the 
jaws  is  continuous  with  the  entoderm  lining  the  mouth  and  throat,  as 
shown  in  Fig.  75,  and  there  is  nothing  to  indicate  w^here  they  join.  To- 
ward the  front  of  the  mouth,  in  either  jaw,  the  epithelium  sends  a  plate-like 
prolongation  into  the  underlying  mesenchyma.     This  is  called  the  dental 

ridge.      There  is  a  continuous  ridge  parallel 
I  with  the  circumference  of  each  jaw,  and  that 

it  is  entirely  ectodermal  is  knowTi  from  the 
study  of  earlier  stages  when  the  oral  plate  is 
still  present.  In  the  diagram,  Fig.  76,  at  A,  a 
part  of  the  ridge  in  the  lower  jaw  and  of  the 
oral  epithelium  from  which  it  grows,  is  repre- 
sented as  free  from  the  surrounding  mesen- 
chyma. The  labial  side  of  the  ridge  is  toward 
the  left  and  the  lingual  side  toward  the  right. 
The  ridge  later  produces  a  series  of  inverted 
cup-shaped  enlargements  along  its  labial  sur- 
face and  these  become  the  enamel  organs. 
There  is  an  enamel  organ  for  each  of  the  ten 
deciduous  or  temporary  teeth  in  either  jaw. 
Within  the  inverted  cups  the  mesenchyma  becomes  very  dense,  producing 
in  each  a  dental  papilla  from  which  the  pulp  and  dentine  are  derived.  The 
enamel  organ  produces  the  enamel,  and  perhaps  controls  the  shape  of  the 
tooth.     The  cement  is  derived  from  the  surrounding  mesenchyma. 

Three  stages  in  the  formation  of  enamel  organs  and  papillae  are 
shown  in  Fig.  76.  The  dental  groove  in  C  is  a  transient  depression  which 
is  relatively  unimportant.  In  D  the  enamel  organs  are  connected  with 
the  dental  ridges  by  slender  necks  of  epithelial  tissue  which  subsequently 
become  severed.  At  about  eleven  weeks  all  the  papillae  and  enamel 
organs  of  the  deciduous  teeth  have  formed.  The  permanent  teeth  develop 
from  similar  organs  and  papillae  which  arise  later;  the  first  molars  are 
indicated  at  five  months,  and  in  embryos  of  six  months  (30-40  cms.) 
all  of   the  permanent  front  teeth  may  be  found.     Their  enamel  organs 


Fig.  75. 
Part  of  a  sagittal  section  of  a  human 
embryo,  to  show  the  position  of 
the  dental  ridges,  D.  R. :  M., 
mouth;  Md.,  mandible;  My., 
maxilla;  N.,  median  nasal  sep- 
tum; P.,  palate. 


TEETH. 


69 


appear  on  the  labial  side  of  the  deep  portion  of  the  dental  ridge,  as  shown 
in  Fig.  77,  but  they  are  on  the  inner  side  of  the  deciduous  teeth.  The 
portion  of  the  dental  ridge  which  is  not  included  in  the  enamel  organs 


Epithelium  of  the  margin 
of  the  jaw. 


Enamel 
oreans. 


Denial  ridge.  //, 

Papillae 


Enamel  organs. 
C 


Necks     of  enamel  organs. 
D 


Fig.  76. — Di.^GRAMS  showing  tHE  Early  Development  of  Three  Teeth,  One  of 

WHICH  IS  shown  IX  Vertical  Section. 

k,  Free  border  of  the  dental  ridge. 

sends  irregular  projections  into  the  mesenchyma  and  becomes  perforated 
and  detached  from  the  oral  epithehum.  Its  remnants  found  in  the  gums 
at  birth  have  been  mistaken  for  glands.  A 
portion  of  the  ridge  extends  beyond  the  necks 
of  the  enamel  organs  for  the  permanent  teeth, 
and  this  has  been  said  to  indicate  the  possibility 
of  a  third  set  of  teeth, — a  possibility  never  real- 
ized in  mammals.  The  second  and  third 
molars  are  formed  from  a  dorsal  or  backward 
extension  of  the  dental  ridge  free  from  the  oral 
epithelium.  The  second  molars  appear  in  a 
child  of  six  months,  and  the  third  or  late  molars 
(wisdom  teeth)  at  five  years.  The  latter  are  not 
at  the  extremity  of  the  dental  ridge  but  are  on 
the  labial  side  of  it,  so  that  there  is  a  theoreti- 
cal possibility  of  fourth  molars. 


;-o.E. 


-D.R. 


&^.0. 


■^: 


Fig.  77- 


Teeth  from  a  Human- 
Fetus  of  30  CMS.  (Modified, 
from  Rose.) 
and  E.  0.,  Enamel  organs  of  a 
deciduous  and  of  a  permanent 
tooth  respectively;  D.  R., dental 
ridge  ;  0.  E.,  oral  epithelium  ; 
P.,  papilla. 


enajmel  orgaxs  axd  enamel. 

The  enamel  organ  is  at  first  a  mass  of  undifferentiated  epithehal  cells, 
but  soon  it  becomes  divisible  into  three  parts  as  shovm  in  Fig.  78.  The 
inner  enamel  cells  are  applied  to  the  dense  mesench>TQal  papilla;  the 
outer  enamel  cells,  continuous  at  the  rim  of  the  cup  with  the  inner  cells, 
are  toward  the  loose  mesenchyma;  and  the  enamel  pulp  fills  the  space 
between  the  outer  and  iimer  layers.  The  outer  enamel  cells  form  a  single 
layer  of  cuboidal  cells,  with  which  some  flattened  cells  of  the  enamel  pulp 
are  in  close  contact.     In  later  stages  the  layer  appears  as  a  feltwork  of 


79  HISTOLOGY. 

flattened  elements.  It  is  in  close  relation  with  the  surrounding  vascular 
mesenchyma,  but  no  blood  vessels  penetrate  it.  The  enamel  pulp  is  at 
first  a  compact  mass  of  ectodermal  cells,  but  by  peripheral  vacuolization 
or  by  the  enlargement  of  intercellular  spaces  it  forms  a  network  con- 
siderably resembling  mucous  connective  tissue  (Fig.  79).  Its  slender 
fibers  have  been  considered  as  elongated  intercellular  bridges.  The  inner 
enamel  cells  form  a  single  layer  of  cylindrical  cells  separated  from  the 
enamel  pulp  by  a  cuticular  plate,  yet  connecting  with  the  pulp  cells  by 
bridges.     Beginning  at  the  summit  of  the  crown  the  inner  enamel  cells 


Thickened 

oral 
epithelium. 


Outer  enamel  cells. 
Enamel  pulp. 
Inner  enamel  cells. 


Neck  of  the 
enamel  organ. 

Free  edge  of 
the  dental  ridge. 


V 


% 


~   Papilla. 


Fig.  7S.— Fro.m  a  Cross  Skction  of  thr  Upper  Jaw,  of  a  Human  E.mbryo 
Five  Months  Old.    X  42. 


produce  cuticular  basal  plates  which  become  long  and  slender,  and  later, 
calcified.  They  extend  from  the  enamel  cells  toward  the  dental  papilla. 
These  are  the  enamel  prisms,  and  the  cells  which  produce  them  are  called 
adamantohlasts  [ameloblasts].  The  formation  of  enamel  prisms  spreads 
from  the  summit  over  the  sides  of  the  crown  and  neck,  but  although  the 
root  is  enveloped  in  the  enamel  organ,  no  prisms  are  formed  there.  The 
inner  enamel  cells  of  the  root  flatten  and  by  disappearance  of  the  enamel 


TEETH.  71 

pulp  they  come  in  contact  with  the  outer  cells.     The  two  layers  form  the 
epithelial  sheath  of  the  root  (Fig.  86). 

The  adamantoblasts  are  columnar  cells  with  elongated  nuclei  toward 
their  outer  ends.  (Since  the  enamel  organ  is  an  inpocketing  of  ectodermal 
epithelium,  it  is  clear  that  the  basal  surfaces  of  the  enamel  cells  are  toward 
the  mesenchyma,  and  the  outer  surfaces  toward  the  enamel  pulp.) 
Diplosomes  have  been  found  near  the  nuclei.     There  are  terminal  bars 

Cuticular  border.     Enamel  prisms.     Cement  substance. 

Calcified, ..uiicalcified  dentine. 


,   Enamel  pulp.                     ■  Rectangle  enclosing;  the  portion 

'                                                '                 Odontoblasts.  Pulp,     of  the  tooth  shown  highly  magni- 

Outer  enamel  cells.                Inner  enamel  cells  fied  in  the  adjoining  part  of  the 

(adamantoblasts).  figure. 

Fig.  79. — Portion  of  a  Longitudinal  Section  of  .an  Incisor  Tooth 

OF  a  Newborn  Kitten.     X  300. 

In  this  section  the  voung  enamel  prisms  have  been'pulled  out  of  their  spaces  in  the  cement  substance. 

(The  cement' of  the  enamel  must  not  be  confused  with  the  cement  which  covers  the  root.) 

and  a  cuticular  border  at  the  basal  surface,  tow^ard  which  the  protoplasm 
contains  granules  which  blacken  with  osmic  acid.  Between  the  cells 
there  is  a  cement  substance.  The  long  colunms  (prisms)  which  grow  out 
from  the  basal  surface  of  the  cells  are  Hkewise  surrounded  by  cement 
substance.  The  columns  at  first  are  not  calcified  [and  are  often  called 
Tomes'  processes];  they  have  a  honey-comb  structure  and  tend  to  split 
into  longitudinal  fibers.  They  may  connect  with  one  another  by  wing- 
like expansions.  Later  both  the  prisms  and  the  cement  substance  become 
calcified,  the  former  increasing  in  diameter  at  the  expense  of  the  cement. 
Eventually  httle  (2-5  %)  or  no  organic  matter  remains  in  the  enamel. 

The  prisms  extend  across  the  enamel  from  its  inner  to  its  outer  surface. 


72 


HISTOLOGY, 


As  they  increase  in  length  the  enamel  layer  broadens.  Their  course  does 
not  remain  straight.  A  vertical  median  section  of  the  enamel  shows  in  its 
middle  part  (Fig.  8i,  c)  alternating  layers  of  prisms  in  cross  and  longitu- 
dinal section.  At  the  borders  of  these  layers  the  prisms  are  in  transition 
from  one  layer  to  the  other.  At  either  end  the  prisms  are  said  to  be  per- 
pendicular to  the  enamel  surfaces,  but  in  the  midst  of  their  course  they 
bend  laterally  in  opposite  directions.  Thus  they  reflect  light  in  such  a  way 
as  to  form  alternating  light  and  dark  bands  (Schreger's  lines)  which  cross 
the  enamel,  and  are  related  to  the  layers  of  prisms  as  shown  on  the  right 
of  Fig.  8i,  c.  The  lines  are  seen  in  reflected  hght.  Contour  lines  (lines 
of  Retzius)  cross  the  prisms  obhquely.  They  are  due  to  pauses  in  the 
enamel  formation,  and  in  poorly  developed  teeth. especially  they  are  planes 
along  which  the  enamel  may  most  readily  be  fractured.     Since  they  often 


Enamel  prisms, 

isolated. 

Fig.  So. — Froivi  a  Child 

AT  Birth. 


Fig.  8i. — a,  Cross  section  of  enamel  prisms  (after  Stolir);  b, 
cross  sections  of  enamel  prisms  (after  Smreker);  c,  Middle 
part  of  the  enamel  from  aground  longitudinal  section  of  a 
canine  tooth  (after  Kolliker).  On  the  right,  seen  in  re- 
flected light,  it  shows  the  light  and  dark  lines  of  Schreger. 


appear  brown  in  sections  they  have  been  ascribed  to  pigment,  but  it  is  said 
that  they  are  air  spaces  in  the  cement.  They  tend  to  be  parallel  with  the 
outer  surface  of  the  enamel,  on  which,  however,  they  terminate  between 
the  httle  encircling  ridges  which  may  be  seen  with  a  hand  lens.  A  few 
contour  lines  but  no  ridges  are  shown  in  Fig.  74. 

In  cross  section  enamel  prisms  are  shown  in  Fig.  81.  They  are  from 
3  to  6  [J.  in  diameter,  sometimes  five  or  six  sided,  but  often  are  concave 
on  one  surface  and  convex  on  the  other,  being  grooved  by  the  pressure  of 
adjoining  prisms.  They  are  said  to  increase  in  diameter  from  the  inner 
toward  the  outer  enamel  surface.  Nodular  enlargements  have  been 
described,  and  transverse  bands  appear  in  isolated  prisms  treated  with 
dilute  acid. 

After  birth  the  tooth  pushes  out  through  the  tissue  of  the  jaw  in 
which  it  is  embedded,  so  that  its  cro\ATi  becomes  exposed.     In  this  process 


TEETH. 


73 


of  eruption  the  outer  enamel  cells  and  the  enamel  pulp  are  broken  through 
and  disappear.  That  portion  of  the  inner  cells  which  is  apphed  to  the 
enamel  prisms  remains  as  an  uncalcified  but  very  resistant  layer  about  i  u 
thick,  the  cuticula  dentis  [Nasmyth's  membrane].  It  may  be  detached  by 
acids  which  dissolve  the  enamel  but  have  little  effect  upon  the  cuticula. 
The  latter,  however,  yields  readily  to  mechanical  erosion,  and  is  soon 
worn  away.  The  enamel  is  the  hardest  portion  of  the  tooth,  surpassing 
the  dentine  which  is  harder  than  bone. 

DENTAL   PAPILLA   AND    DENTINE. 

The  dental  papilla  has  already  been  described  as  a  dense  mass  of 
mesenchyma  enclosed  and  probably  moulded  by  the  enamel  organ.  Its 
cells  branch  and  anastomose,  producing  fibrils.     The  cells  next  to  the  inner 


Fig.   S2. — The  Development  of   Dentine 

IN  Pig  Embryos.     (After  v.  Korff.) 
d.,  Calcified  dentine;  e.  C,  inner  enamel  cells  ; 

f.,  fibrous   g:round   substance  of  dentine; 

od.,  odontoblasts;    p.,  mesenchymal  pulp 


Fig.  S3. 

Six  odontoblasts  with  dental  fibers,  f.    p.,  pulp  proc- 
esses.    From  the  pulp  at  birth.     X  240. 


enamel  layer  become  elongated  as  shown  in  Fig.  82,  A,  and  soon  constitute 
a  simple  epithelioid  layer  as  in  B.  Between  them  there  are  groups  of  fibrils 
which  spread  beneath  the  enamel  layer.  Calcareous  granules  are  de- 
posited between  the  fibrils  and  produce  the  matrix  of  the  dentine.  The 
elongated  cells  which  are  comparable  with  osteoblasts  are  called  odonto- 
blasts. Unlike  the  former  they  never  become  buried  in  the  matrix,  but 
remain  on  its  inner  surface.  Long  processes  extend  from  the  odonto- 
blasts radially  through  the  dentine  as  seen  in  the  isolated  cells  in  Fig. 
83.  These  processes  are  lodged  in  the  dental  canaliculi  and  are  called 
dental  fibers  [Tomes'  fibers].  As  in  bone  the  canaliculi  have  an  incom- 
pletely calcified  lining  which  resists  acids.  [The  canaliculi  of  the  teeth 
have  therefore  been  described  as  bounded  by  Neumann's  membrane.] 
They  follow  a  wavy  or  spiral  course  from  the  outer  to  the  inner  surface  of 
the  dentine,  often  being  S-shaped  as  seen  in  median  longitudinal  sections. 


74 


HISTOLOGY. 


Their  diameter  increases  toward  the  inner  surface  where  it  is  from  2  to  4  n- 
They  branch  freely,  as  shown  in  Figs.  84  and  85,  and  terminate  blindly 
or  by  connecting  with  neighboring  canaliculi.  Sometimes  they  are  pro- 
longed into  the  enamel  for  a  short  distance;  they  may  end  abruptly  as  if 
the  terminal  part  had  been  destroyed  or,  in  the  permanent  teeth,  the 
enamel  may  form  knobs  invading  the  dentine.  Ordinarily  the  contact 
between  enamel  and  dentine  is  smooth. 

The  calcification  of  dentine  begins  shortly  before  the  formation  of 
enamel  and  spreads  from  the  cro\Mi  over  the  neck  and  root  (Fig.  86). 
The  calcified  portion  increases  in  thickness,  and  contour  lines,  indicative 
of  stratification,  are  sometimes  seen.  Near  the  enamel  there  are  large 
irregular  spaces  of  uncalcified  matrix  which  occur  in  the  course  of  the 


Enamel  prisms. 


Dentine. 


Enamel. 


Fig.  84. — From  a  Longitudinal  Section  of  the  Lat- 
eral Part  of  the  Crown  of  a  Human  Molar 
Tooth.     X  240. 

1,  Dental  canaliculi,  some  extending  into  the  enamel;  2, 
dental  globules  projecting  toward  the  interglobular 
spaces,  3. 


Cement. 


Fig.  85. — Fro.m  a  Longitudin.\l  Sec- 
Tio.v  OF  the  Root  of  a  Human 
Molar  Tooth.     X  240. 

1,  Dental  canaliculi  interrupted  by  a  stra- 
tum with  many  small  interglobular 
spaces,  2.  3,  bone  lacunae  and  canali- 
culi. 


contour  lines  if  such  are  apparent.  The  spaces,  which  in  section  suggest 
bone  lacunae,  are  bounded  by  rounded  masses  of  calcified  dentine,  and 
are  therefore  called  inter  globular  spaces  (Fig.  84).  The  reason  for  their 
persistence  is  unknown.  The  interglobular  spaces  of  the  root  are  much 
smaller  and  more  numerous  than  in  the  crown.  As  seen  in  Fig.  85  they 
occur  in  a  layer  of  dentine  not  far  from  its  outer  surface,  and  because 
with  low  magnification  they  appear  as  dots  this  layer  is  sometimes  named 
the  "granular  layer."  The  compact  dentine  beyond  it  is  closely  joined  to 
the  substantia  ossea,  their  canaliculi  having  been  said  to  communicate. 
The  epithehal  sheath  which  bounded  the  dentine  in  early  stages  becomes 
reduced  to  fragments,  thus  allowing  the  cement  and  dentine  to  unite. 

The  pulp  of  the  adult  tooth  is  a  very  vascular  connective  tissue  of 
embryonic  type.     It  suggests  reticular  tissue  since  its  fibrils  do  not  form 


TEETH. 


/D 


coarse  bundles,  and  the  cell  processes  remain  evident.  Elastic  elements 
are  absent.  The  small  arteries  entering  the  apical  foramen  send  capillaries 
close  to  the  odontoblasts,  but  they  do  not  enter  the  dentine.  There  are  no 
lymphatic  vessels  in  the  pulp.     The  medullated  dental  branches  of  the 


Dental  sac. 
Outer  layer.        Inner  layer. 
\  \- 


Outer  enamel  cells. 


Enamel  pulp. 


Inner  enamel  cells. 


V-- 


Enamel. 


/^; 


Dentnie 


Epithelial 
sheath. 


Odontoblasts 


Dental  papilla  (f utui  e  pulp) 


Blood  vessel    / 
Bony  trabecula  of  the  lower  jaw. 


Fig.  86. — Longitudinal  Section  of  a  Deciduous  Tooth  of  .\  Newborn  Dog.    X  42. 
The  white  spaces  between  the  inner  enamel  cells  and  the  enamel  are  artificial,  and  due  to  shrinkage. 

alveolar  nerves  pass  through  the  foramen,  lose  their  sheaths  and  form  a  loose 
plexus  beneath  the  odontoblasts,  between  which  they  terminate  in  free 
endings.  Odontoblasts  persist  throughout  the  life  of  a  tooth,  and  in  case 
of  disease  or  injury  they  may  deposit  dentine  as  a  reparative  process. 


76 


HISTOLOGY. 


DENTAL    SAC,    CEMENT   AND    ALVEOLAR   PERIOSTEUM. 

The  papilla  and  enamel  organ  together  are  surrounded  by  loose 
mesenchyma  extending  to  the  oral  epithelium  and  to  the  bone  trabeculae 
of  the  developing  jaws,  as  shown  in  Fig.  87.  The  portion  of  mesenchyma 
between  the  trabeculae  and  the  teeth  forms  the  so-called  dental  sacs. 
Toward  the  enamel  organ  the  sac  is  a  vascular  and  \'ery  loose  tissue  (Fig.- 
86)  which  may  form  elevations  between  projections  of  the  outer  enamel 
layer.     The  peripheral  part  of  the   sac   is  much  denser.     After  birth, 


Cross  section  of  the 
orbicularis  oris  muscle. 


Labial  sland. 


Dental  ridge. 


Enamel 
oro-an. 


Enamel. 


Dentine. 
Pulp. 
Bone. 

Fig.  87.— \'krtical  Section  through  the  Lip  and  Jaw  of  a  Human  Fetus 
OF  Si.\  AND  A  Half  Months.    X  9- 

but  before  the  eruption  of  the  teeth,  the  sac  surrounding  the  root  produces 
the  cement  or  substantia  ossea.  This  is  a  layer  of  bone,  containing  typical 
lacunae  and  canahculi  and  penetrated  by  many  uncalcified  connective  tissue 
fibers  (Sharpey's  fibers).  These  may  be  so  numerous  as  to  suggest  the 
columnar  appearance  of  enamel.  Their  direction  is  generally  radial. 
Lamellae  in  the  cement  are  parallel  with  the  surface  of  the  root.  Haversian 
canals  are  absent  except  in  the  outer  part  of  the  cement  of  old  teeth. 

As  the  tooth  grows  and  fills  the  alveolar  socket  in  the  jaw  bone,  the 
dental  sac  is  reduced  to  a  vascular  fibrous  layer,  continuous  with  the 
connective  tissue  of  the  gums  at  the  neck  of  the  tooth.     Elastic  fibers  are 


MUSCLE.  77 

absent.  It  is  a  single  layer  serving  as  the  periosteum  of  the  cement  on 
one  side  and  of  the  alveolus  on  the  other  and  being  intimately  joined  to  both 
bones  by  Sharpey's  fibers.  It  is  named  the  alveolar  periosteum  [peridental 
membrane].  Its  numerous  blood  vessels  are  branches  of  those  which  enter 
the  apical  foramen  together  with  vessels  from  the  gums  and  perhaps  from 
the  mandible  and  maxilla.  Its  nerve  endings  are  the  terminations  of 
branches  from  the  dental  and  alveolar  nerv^es.  Lymphoid  tissue  has  been 
found  in  the  gums,  but  apparently  it  does  not  extend  into  the  alveolar  peri- 
osteum. 

MUSCLE   TISSUE. 

Contractility  is  a  fundamental  property  of  protoplasm.     Muscle  cells 
are  those  in  which  the  contractile  function  has  become  predominant. 
They  are  elongated  cells  containing  fibrils  parallel  with  their  long  axes. 
By  the  shortening  of  these  fibrillated  cells  muscular  action  results.     Em- 
bryologically  muscles  arise  either  from  mesenchyma  or  from  epithehum. 
Mesenchyma  produces  two  types  of  muscle,  smooth  (non- striated,  involun- 
tary) and  cardiac  (the  striated,  involuntary  mus- 
cle of  the  heart) .    Mesodermal  epithelium  pro-        f        i^  -      - 
duces  one  type,  the  striated  voluntary  skeletal        "^  ^P'- 
muscles,  ordinarily  called  striated.     In  the  in-        -                              V 
vertebrates  ectodermal  and  entodermal  epithe-       -~^                     1    "^    ' 
lia  also  produce  muscle  cells.    In  mammals  the                 --    , 
muscle  fibers  of  the  sweat  glands  are  generally      ^ 
recognized  as  ectodermal,  and  some  in  the  iris      sj     <m       _,    '^'^ 
have  been  described  as  such;  entodermal  mus-      rrrtr^^^-C       -r^-^^^  era. 
cles  have  not  been  observed.                                     ^^  -  '  -^^  i 

The   three  principal   classes  of  muscles, 

smooth,  cardiac,  and  striated,  may  be  described  ^    l.nL 

in  turn.  o      d 

&    c 

Smooth  Muscle.  O     ,  cd     o   -'^  d. 

Smooth  muscle  develops  around  the  large       5fV^      ..,-'^'- 
lymphatic  and  blood  vessels:  around  the  intes-      fig.  ss.— from  a  cross  sectiox 

■^       -^  OF  THE    Oesophagus   of   an 

tinal  canal,  including  the  principal  gland  ducts  ^s  mm.  pig. 

epi.,  Epithelium;  b.  m.,  basement 

openmg  mto  it  and  the  large  respiratory  tubes;  membrane;  c.  t.,  connective 

■'■  °  CI  i.  J  tissue;  c.  m.,  circular  smooth 

also  around  the  bladder  and  ureters,  the  uterus  muscle  cut  lengthwise ;  n.  c., 

'  nerve  cells;  I.  m.jlong-itudmal 

and  ducts  of  the  genital  system;  and,  finally,  in  s'"°°'h  muscle  cut  across, 

connection  with  the  hairs,  in  the  capsule  of  the  spleen,  and  in  other  minor 
places.     In  general  terms,  it  forms  the  musculature  of  the  viscera. 

The  development  of  smooth  muscle  may  be  studied  in  a  cross  section 
of  an  1 8  mm.  pig  embryo  (Fig.  88).     The  stratified  entodermal  epithehum 


.J  n.c. 


75  HISTOLOGY. 

which  Hnes  the  oesophagus,  a  part  of  which  is  shown  in  the  figure,  is  seen 
to  be  surrounded  by  mesenchymal  tissue  in  which  the  smooth  muscle  cells 
are  being  differentiated.  There  is  a  layer,  cm.,  in  which  the  cells  have 
become  spindle-shaped,  and  since  they  are  parallel  and  close  together, 
they  form  a  band  encircling  the  oesophagus.  Outside  of  this  there  is  a 
broader  layer  of  elongated  cells,  1.  m.,  all  nmning  lengthwise  of  the  oesoph- 
agus and  therefore  cut  across  in  this  section.  This  layer  of  longitudinal 
muscle  passes  into  mesenchymal  tissue  on  the  outside.  The  figure  illus- 
trates that  smooth  muscle  cells  are  elongated  mesenchymal  cells,  gener- 
ally parallel  and  ar- 
ranged in  layers. 
In  the  embryonic 
stage  they  are  con- 

sci.E  Fibers  from  the  Small  Intestine  -rioz-forl       K-ir      ■r\i-r\fr^ 

OF  A  Frog.    X  240.  necLcu       oy     prOLO- 

plasmic  processes. 

Smooth  muscle  cells  in  the  adult  may  occur  singly  or  in  the  form  of 
interlacing  networks.  Generally  they  are  in  layers  and  so  closely  packed 
that  separate  cells  are  hard  to  follow.  ISIoreover  they  often  extend  be- 
yond the  planes  of  the  section  so  that  only  portions  of  them  are  included 
in  the  specimen  examined.  If  a  piece  of  fresh  tissue  is  treated  with  a 
35  %  aqueous  solution  of  potassium  hydrate  or  20  %  nitric  acid,  the  cells 
may  be  shaken  apart,  and  appear  as  in  Fig.  89.  They  vary  in  length  from 
0.02  mm.  in  some  blood  vessels  to  0.5  mm.  in  the  pregnant  utems;  in  the 
intestine  they  are  said  to  be  about  0.2  mm.  Their  width  ranges  around 
0.005  ^^-  (5  A*)-  They  are  fusiform  or  cylindrical, 
rarely  being  branched  as  has  been  recorded  for  muscle 
cells  in  the  bladder,  the  ductus  deferens,  and  the  aorta 
(Fig.  156,  p.  131)- 

The  nucleus,  situated  near  the  center  of  the  cell, 
is  cylindrical,  with  its  chromatin  in  a  network  and  in     ^^^u  9°-  —  ^^'\\^^  °^ 

J  '  Smooth  Muscle 

masses  lining  the  nuclear  membrane.   In  favorable  prep-  aLt1s\%  ©"y^AVoc^ 

arations  it  has  been  observed  to  contain  several  nucleoli, 
and  a  diplosome  has  been  found  just  outside  of  its  longitudinal  border. 
When  the  muscle  cell  contracts  the  nucleus  shortens  and  may  be  bent 
or  spirally  twisted.  Fig.  90.     (Such  nuclei  have  been  interpreted  as  distor- 
tions of  resting  nuclei  caused  by  the  contraction  of  neighboring  cells.) 

The  protoplasm  of  the  smooth  muscle  cells  early  produces  coarse 
fibrils  called  border  fibrils  [myoglia],  since  they  tend  to  be  at  the  periphery 
of  the  cell.  They  are  said  to  extend  from  cell  to  cell,  which  is  made  pos- 
sible by  the  syncytial  arrangement  of  mesenchyma.  In  one  interesting 
but  unique  instance,  the  fibrils  from  the  mesentery  of  a  salamander  showed 


SMOOTH    MUSCLE. 


79 


■'-  c 


/  '^^^ 

End  of  a  muscle  fiber.  Nerve  cell. 

Fig.  91. — Apparent  Intercellular  Bridges  of  Smooth  Muscle  Fibers. 
A,  Transverse  section  of  the  intestine  of  a  rabbit.     B,  Longitudinal  section  of  the  intestine 
of  a  guinea  pig.     X  420. 

alternating  light  and  dark  bands,  very  distinct  in  photographs.  The 
fibrils  of  cardiac  and  striated  muscles  are  always  banded  in  this  way. 
Some  investigators  consider  that  the  border 
fibrils  are  the  contractile  elements.  Others  hold 
that  by  their  elasticity  they  cause  the  muscle 
cells  to  elongate  after  contraction,  thus  being  an 
obstacle  to  contraction.  The  elongation  of  the 
relaxed  muscles,  either  in  the  blood  vessels  or  in 
the  intestinal  wall,  may  be  accomplished  by  the 
pressure  of  the  contents  of  these  organs,  or  by  the 
elastic  connective  tissue  which  is  outside  of  the 
muscle  cells.  In  the  endoplasm  of  smooth  muscle 
cells,  and  thus  surrounded  by  the  border  fibrils, 
minute  inner  fibrils  have  been  described  and  said 
to  be  contractile.  Among  them  is  the  unaltered 
protoplasm.  Where  the  fibrils  diverge  to  pass 
around  the  nucleus,  that  is,  at  the  ends  of  the 
nucleus,  the  granular  protoplasm  is  most  readily 
distinguishable.  In  the  intestine  it  has  been 
observed  to  contain  pigment.  Surrounding  the 
smooth  muscle  cells  there  is  probably  a  dehcate 
cell  membrane,  but  the  nature  of  the  structures 
observed  is  still  under  discussion.  The  cell  mem- 
brane of  a  muscle  cell  is  called  a  sarcolemma ;  its 
protoplasm  is  named  sarco plasm;  and  the  entire 
cell  is  called  a  muscle  fiber.  Fibril  is  applied  to  the 
filaments  within  the  fibers. 

Smooth  muscle  cells  are  bound  together  so 
that  they  may  act  in  unison.    They  may  be  joined  end  to  end  by  the  border 
fibrils.    Protoplasmic  bridges  have  been  described  between  them  (Fig.  91). 


Fig.  92. — Fierols  Tissue  ix 
Relation  WITH  Smooth 
Muscle  Fibers,  from  the 
Bladder  of  a  Pike.  (After 
Prenant.) 

C,  Connective  tissue  network  ; 
n.,  p.,  f.,  nucleus,  granular 
protoplasm,  and  fibrillar  pro- 
toplasm of  a  muscle  Cell. 


So  HISTOLOGY. 

They  are  certainly  closely  invested  by  connective  tissue  membranes  or  net- 
works (Fig.  92),  consisting  of  white  and  elastic  elements  and  extending  from 
cell  to  cell.  These  may  be  formed  from  the  protoplasmic  processes  of  the 
mesenchymal  muscle  cells,  or  from  distinct  interspersed  connective  tissue 
cells.  Tissue  spaces  exist  in  this  network  between  the  muscle  fibers. 
The  loose  muscular  coat  of  the  blood  vessels  in  the  umbihcal  cord  is  a 
particularly  favorable  place  for  the  study  of  fibrous  tissue  in  relation  to 
smooth  muscle. 

In  ordinary  sections  the  student  should  recognize  smooth  muscle 
by  the  parallel  arrangement  of  its  cells,  with  which  the  nuclei  correspond, 
and  by  the  protoplasmic  appearance  of  muscle  substance  as  compared 
with  fibrous  connective  tissue.  In  doubtful  cases  Mallory's  connective 
tissue  stain  may  be  used,  making  the  muscle  substance  red  and  the  white 
fiber  blue.  In  cross  section  smooth  muscle  appears  as  in  Fig.  93.  Since 
the  cells  taper  the  sections  near  their  ends  are  smaller  than  the  others. 
Only  those  cut  near  their  centers  show  nuclei.     Between  groups  of  muscle 

cells   there  are  generally  bands  of 
„,,,,,    ,„,„,..       connective    tissue   containing    lym- 

Conneclive  tissue      \\  \L--A(Io  )w'''i\-<^^ 

septum.         0  P  ^/^^T^^W^f       phatic  and  blood  vessels,  and  nerves 

which  terminate  in  contact  with  the 


Smooth  muscle  fibers')  W:^Vfi(|VAj>^^         cclls  in  a  manner  to  be  considered 

and  nuclei  in  cross  ^      -Il^,A->— ^  ^^>?_^'^  i  t        i  -i   •  i  i 

section.  J  __|4^2;^4;>5,^,/  later.    In  describmg  smooth  muscle 

c        V  ^^  ^„=-  ,-,  A*  the   student   should  always   record 

Fig.  93. — Section  of  the  Circular  Muscle  -' 

Coat  of  the  Human  Intestine.    X  560.  whether   it    IS   circular,  longitudinal, 

or  obHque  in  relation  to  the  organ  of  which  it  forms  a  part.  This  relation 
is  independent  of  the  plane  in  which  the  organ  has  been  sectioned,  and  in 
many  small  sections  it  cannot  be  determined  from  observation.  He  should 
add  the  way  in  which  the  fibers  are  cut,  whether  lengthwise  or  across,  and 
this  depends  entirely  on  the  way  in  which  the  sections  happened  to  be  made. 
It  can  always  be  observed  in  the  specimen.  Thus  in  Fig.  88  the  student 
should  observe  an  inner  layer  of  muscle  fibers  cut  lengthwise  and  an  outer 
layer  cut  across.  If  he  knows  that  the  inner  layer  of  intestinal  muscles 
is  generally  circular,  and  the  outer  layer  is  longitudinal,  he  infers  that 
Fig.  88  is  from  a  cross  section  of  the  oesophagus.  If  the  oesophagus  had 
been  split,  the  inner  circular  fibers  would  have  been  cut  across  and  the 
outer  ones  cut  lengthwise.  Being  told  that  Fig.  93  represents  the  circular 
layer  of  muscle,  he  can  state  whether  it  is  from  a  transverse  or  a  longitudinal 

section  of  the  intestine. 

Cardiac  Muscle. 

Cardiac  muscle  begins  as  a  mesenchyma  with  very  broad  protoplasmic 
connections  between  its  cells.  This  syncytial  condition  is  retained  in  the 


CARDIAC    MUSCLE. 


adult,  cardiac  muscle  being  a  network  of  broad  protoplasmic  bands,  in 

and  near  the  centers  of  which  nuclei  are  situated  at  irregular  intervals 

Lateral  union.  (Fig.  94).     The  interccllular  spaces 

are  reduced  to  clefts  occupied  by  a 

...  small  amount  of  connective  tissue, 

'*    '~  which  is  either  a  part  of  the  original 


Capil 
larv. 


ft 


~  — as^^ 


Nucleus  of    Nucleus  of 
a  muscle     a  connective 
fiber.  tissue  cell. 


Intercalated 
disc. 


Fig.  94. — Fro.m  a  Longitudinal  Section  of 
A  Papill.\rv  Ml'scle  of  the  Human 
Heart.    X  360. 


Fig.  95.— Part  of  the  Muscular  Sy.ncytiu.m 
FROM  THE  Heart  of  a  Duck  Embryo  of 
3  D.\YS.  (/./.  Heidenhain ,  from  McMurrich's 
'■  Embr\-ology. ") 


mesench}'ma  or  a  later  ingrowth  accompanying  the  blood  vessels. 

The  protoplasm  of  cardiac  muscle  contains  longitudinal  fibrils 
in  development  they  are  feiv  in  number  and  situated 
near  the  periphery  of  the  bands  of  protoplasm.    They 
extend  for  considerable  distances  through  the  syncytium  \ 

regardless  of  cell  areas  (Fig.  95).      Their  origin  is  a  % 

subject  for  speculation.     It  has  been  suggested  (i)  that 
they  are  bundles  of  ultra-microscopic  molecular  fhrils; 
(2)  that  they  develop  by  the  coalescence  of  granules 
in  the  hyaloplasm  between  the  reticular  network   of        epi^ 
protoplasm;  and  (3)  that  they  are  parts  of  this  network,        iW'"^ 
supposed  to  be  retractile,  which  is  irregularly  arranged        y'    _  ;. 
in  ordinary  cells  but  which  in  muscle  cells  has  acquired 
rectilinear  meshes.     At  first  homogeneous,  they  soon 
become  marked  by  alternating  hght  and  dark  bands. 
They  increase  in   number  by  longitudinal   splitting. 
6 


Early 


-7i 


Fig.  96. — Muscle  Fiber 

of  x  Frog.     X  240. 
f.,  Fibrillae  ;  k.,  nucleus. 


The   protoplasm 


02  HISTOLOGY. 

becomes  nearly  full  of  these  fibrils,  so  arranged  that  their  light  and  dark 
bands  appear  to  form  continuous  stripes  across  the  muscle  fiber  (Fig.  94). 
That  the  transverse  striations  are  optical  effects  is  sho^^Ti  by  the  readiness 
with  which  they  may  be  broken  up  by  the  separation  of  the  longitudinal 
fibrils  (Fig.  96).  The  dark  bands  stain  more  deeply  than  the  light  ones, 
which  perhaps  is  not  due  to  chemical  differences  but  is  because  they  are 
denser,  containing  less  water.  In  polarized  light  the  dark  bands  are 
"doubly  refractive"  or  anisotropic  and  the  fight  ones  are  "singly  refrac- 
tive" or  isotropic. 

The  finer  structure  of  the  fibrils  such  as  occur  both  in  cardiac  and  in 
the  skeletal  muscles,  is  sho^n  in  the  diagram.  Fig.  97.  The  light  band 
is  bisected  by  a  slender  dark  one  said  to  be  continuous  from  one  side  of 
the  fiber  to  the  other,  thus  connectmg  the  fibrils  with  one  another.  Since 
such  a  transverse  membrane  is  not  present  from  the  first  it  has  been 
suggested  that  it  forms  by  lateral  outgrowths  of  the  fibrils.  It  is  named 
the  ground  membrane  of  Krause,  and  is  always  designated  by  the  letter 
Z.     The  light  band  is  /.     The  large  dark  band  seen  with  ordinarv  lenses 


Fig.  97. — Diagram  of  Muscle  Striations. 
The  fibrils  consist  of  alternating  dark  bands,  q,  and  light  bands,  j.    j  is  traversed  bv  the  ground  mem- 
brane z,  and  q  by  the  median  membrane   m.     In  the  right  of    the  three    muscle   segments  shown 
in  the  figure  the  bands,  n,  have  been  drawn.     (The  portion  of  j  between  n  and  z  is  designated  e.) 


is  called  Q.  It  grows  hghter  toward  its  middle  part  where  it  is  sometimes 
crossed  by  the  median  membrane  of  Heidenhain,  M.  This  is  thought  to 
be  similar  to  the  ground  membrane  Z,  but  more  dehcate.  The  light 
portion  of  Q  through  which  it  passes  is  designated  H.  In  some  highly 
developed  muscles  of  insects  a  dark  band  N  is  found  in  /.  It  is  of  un- 
certain nature.  The  fiber  as  a  whole  is  divided  by  the  ground  membranes 
which  cross  it,  into  a  series  of  similar  compartments  called  muscle  segments 
(sarcomeres).  Additional  sarcomeres  may  be  formed  at  the  ends  of 
muscle  fibers;  it  has  not  been  found  that  the  median  membrane  can  become 
a  ground  membrane,  thus  producing  two  segments  from  one. 

The  contraction  of  muscles  corresponds  in  its  rate  with  the  complexity 
of  the  striae.  Thus  smooth  muscles  which  are  non-striated  contract 
slowly.  The  more  rapidly  actmg  muscles  of  some  invertebrates  have 
banded  fibrils  but  lack  the  orderly  arrangement  which  produces  transverse 
striations.  The  highest  development  of  striated  structure  is  perhaps  in 
the  wing  muscles  of  insects  which  contract  with  great  rapidity.     As  the 


CARDIAC    MUSCLE. 


83 


muscle  cell  contracts  it  broadens,  and  shortens,  even  to  one  tenth  of  its 
length  when  at  rest  (Prenant).  The  ground  membranes  approach  one 
another  (Fig.  98).  It  has  been  said  that  by  a  transfer  of  light  substance 
to  the  dark  the  staining  reactions  are  reversed,  but  this  has  been  denied. 
The  retreat  of  the  protoplasm  into  capillary  spaces  between  the  dark 
fibrils  has  been  described.  The  process  is  kno"^\Ti  to  be  most  complex, 
involving  physical  (electrical)  and  chemical  changes  which  are  but  im- 
perfectly expressed  in  the  histological  pictures.  With  prolonged  activity 
„  the  muscle  nuclei  are  said  to  shrink 

fand  to  stain  less  deeply. 
In  ordinary  specimens  of  cardiac 
^,  c  muscles  the  student  will  observe  only 

the  alternating  hght  and  dark  bands, 
with  possibly  the  ground  membrane 
Z.  On  changing  the  focus  the  dark 
bands  may  appear  hght  and  vice 
versa,  but  in  the  proper  focus  for  ad- 
jacent nuclei  and  connective  tissue, 


iP"^ 


Fig.  99.— Intercalated  Disc  (d)  from  Human 
Cardiac  Muscle,  Stained  with  Thiazin 
Red  and  Toluidin  Blue.  The  Ground 
IMembranes  are  Lettered  z.  (Heiden- 
hain.) 


Fig.  98.— Fibrils  fro.m  the  Wing  JNIuscles 

OF  a  Wasp.     (Schafer.) 
A,  Contracted  ;    B,  stretched  ;    C,  uncontracted. 
■    The  dark  bands  are  bisected  by  the  light 
stripes  ( H ) ,  but  they  do  not  show  the  median 
membranes  (M).  • 

the  bands  appear  as  has  been  described.  x\t  irregular  intervals,  in  cardiac 
muscle  only,  transverse  lines  of  another  sort  may  be  found,  called  inter- 
calated discs  and  formerly  loiown  as  cement  lines. 

Intercalated  discs  are  seen  in  Fig.  94,  and  as  pictured  by  Prof.  Heid- 
enhain,  in  Fig.  99.  He  describes  them  as  deeply  staining  plates  almost 
invariably  not  as  wide  as  a  muscle  segment.  The  segment  in  the  human 
heart  is  2  ,0-,  whereas  the  intercalated  discs  vary  from  i  to  1.7  ,".  A  disc 
may  extend  straight  across  a  fiber,  or  it  may  be  interrupted  so  as  to. form 
a  succession  of  steps,  usually  from  two  to  four.     The  discs  are  always 


84  HISTOLOGY. 

connected  with  ground  membranes.  It  may  be  said  that  here  and  there 
within  the  cardiac  muscle  two  successive  ground  membranes  are  closer 
together  than  usual  and  the  fibrils  in  crossing  such  an  interval  become 
expanded  and  more  stainable,  thus  making  an  intercalated  disc.  The 
discs  have  been  variously  interpreted,  for  example,  as  locally  contracted 
segments ;  as  lines  where  the  fibrils  are  inserted  and  upon  which  they  may 
pull  in  contracting;  or  as  places  where  the  fibrils  may  grow  to  form  new 
segments,  being  comparable  with  the  unhanded  embryonic  fibrils.  The 
older  idea  that  they  are  cell  boundaries,  either  cement  fines,  or  protoplasmic 

bridges,  is  supported  by 

-^\  ^     "^""^^  ^^^    tendency    of    heart 

.^  ^  muscle  to  rupture  along 

,1,  «         \  their  course.    They  mark 

*  off  irregular  spaces,  how- 

,  ever,    some    containing 

v'  more  than  one  nucleus, 

N 

\  capii-         afid     others     non-nucle- 
^,     anes.         atcd.     Intercalated  discs 
*     :>   ^-'"  should   be  distinguished 

•;^  '  from    the    cut    edges   of 

'  fiber,    made    where    a 

branch  of  the  syncytium 
extending  toward  the  ob- 
server, passed  out  of  the 
plane  of  section. 

The  nuclei  of  cardiac 
muscle  are  round  or  oval 

hiido-        Elastic     Nucleus     Cross     Nucleus     Nuclei  of  coniiec- 
liieliuni.        fibers.         of  a       sections      of  a  live  tissue  cells.  and    arC   foUnd    near    the 

connective      of         muscle 

''s«"«    ""'scie    fiber.  Central  axes  of  the  fibers. 

cell.        fibers. 

Fig.  100.— From  a  Cross  Sectio.n  of  a  Papii.i.arv  Muscle         As   the  fibrils   Spread   OUt 
OK  THE  Human  Heart.    X  360. 

to  pass  around  them, 
often  a  considerable  quantity  of  granular  protoplasm  may  be  seen,  con- 
taining fat  droplets  and  pigment  granules  which  increase  with  age.  A 
delicate  membrane  (sarcolemma)  has  been  described  as  surrounding  the 
cardiac  fibers,  and  in  it  the  ground  and  median  membranes  are  said  to 
terminate.  Some  of  the  clefts  in  cardiac  muscle  are  protoplasmic  (sarco- 
plasmic) intervals  between  bundles  of  fibrils.  Others,  bounded  by  the 
sarcolemma,  are  spaces  which  contain  capillary  vessels  closely  apphed  to 
the  muscle.  Probably  always  a  little  connective  tissue  intervenes  between 
the  vessel  and  sarcolemma.  The  connective  tissue,  which  is  more  abundant 
toward  the  surfaces  of  the  heart,  contains  tissue  spaces  and  the  nerves 


/ 


; 


STRIATED    MUSCLE.  85 

which  terminate  in  contact  with  the  cardiac  muscle  fibers.  Lymphatic 
vessels  are  found  in  the  larger  layers  and  bands  of  connective  tissue,  but 
they  end  before  penetrating  between  the  separate  fibers. 

Although  the  cardiac  muscle  fibers  form  a  network,  they  are  in  layers, 
each  having  one  general  direction.  Since  the  predominant  direction  varies 
in  different  parts  of  a  single  section  it  is  possible  to  find  places  where 
the  fibers  are  mostly  cut  lengthwise  as  in  Fig.  94,  and  others  where  they 
are  cut  across  (Fig.  100).  Here  transverse  bands  and  intercalated  discs 
cannot  be  seen.  The  nuclei  surrounded  by  some  protoplasm  are  near 
the  centers  of  the  fibers.  The  fibrils  cut  across  appear  as  dots  which  shift 
about  but  do  not  disappear  on  focusing,  since  even  in  thin  sections  they 
are  not  granules  but  short  perpendicular  rods.  They  are  arranged  in 
radiating  lines,  or  in  clumps  known  as  muscle  columns.  Close  to  the  inner 
lining  of  the  heart  the  muscle  fibers  may  be  imperfectly  developed,  con- 
taining only  a  peripheral  ring  of  fibrils.  These  fibers  (of  Purkinje) 
are  abundant  in  the  sheep  but  are  infrequent  in  man. 

COMPARISON    OF    MESENCHYMAL   MUSCLES. 

Smooth  muscles  are  slender  mesenchymal  cells  containing  contractile 
fibrils  which  are  not  banded.  The  cells,  surrounded  by  a  fibro-elastic 
network,  are  generally  closely  associated  in  layers.  If  the  border  fibrils 
actually  pass  from  cell  to  cell,  as  has  been  said,  then  smooth  muscle,  like 
other  muscle,  is  syncytial  in  nature. 

Cardiac  muscle  is  a  syncytium  of  mesenchymal  origin,  consisting  of 
broad  approximately  parallel  branches.  It  contains  banded  contractile 
fibrils  not  limited  by  cell  areas.  It  is  distinguished  from  smooth  muscle 
by  its  cross  striations  and  by  the  width  of  its  fibers;  and  from  striated 
(voluntary)  muscle  by  its  mesenchymal  origin,  the  branching  of  its  syncy- 
tium, the  central  position  of  its  nuclei,  and  the  possession  of  intercalated 
discs. 

Striated  Muscle. 

Striated  muscle,  as  the  term  is  ordinarily  used,  does  not  include  the 
striated  cardiac  muscle,  but  only  the  striated  muscle  which  develops  from 
the  epithelium  of  the  mesodermic  segments  [protovertebrae].  The  segments 
form  a  series  of  paired  masses  of  cells  found  on  either  side  of  the  medullary 
tube.  They  have  been  briefly  described  on  page  22.  At  first  they  are 
epithelial  structures  bounding  a  part  of  the  coelom  or  body  cavity.  Later 
they  lose  their  connection  with  the  coelom  (Fig.  21)  and  become  rounded 
masses  of  cells,  each  mass  enclosing  a  cavity.  From  the  median  side 
of  the  segment,  near  its  ventral  border,  a  stream  of  mesenchymal  cells  is 


86 


HISTOLOGY. 


fit 


3    0 


given  ofF,  which  surrounds  the  notochord  and  produces  the  vertebral 
cartilages  and  intervertebral  discs.  It  also  extends  around  the  medullary 
tube.  This  stream  of  cells  is  called  the  sclerotome.  The  rest  of  the  seg- 
ment becomes  flattened  and  plate  like,  by  the  approximation  of  its  lateral 
and  medial  walls.  Thus  the  central  cavity  is  obliterated.  Fig.  loi,  i, 
shows  a  cross  section  of  such  a  segment.  Its  medial  layer  is  called  the 
muscle  plate  or  myotome.  Here  the  cells  multiply  rapidly  by  mitosis  and 
become  elongated  lengthwise  of  the  embryo.  They  are  called  myoblasts 
and  become  the  striated  muscle  cells.  The  lateral  layer  of  the  segment, 
named  the  cutis  plate  or  dermatome,  was  supposed  to  form  only  mesen- 

chyma  which  became  the  deeper  part  of  the 
skin.  It  also  forms  striated  muscles,  however, 
and  in  the  pig  it  is  said  to  be  concerned  only 
with  muscle  formation.  The  elongated  cells 
of  the  myotome  become  separated  from  one 
another  by  mesenchyma,  containing  blood  ves- 
sels. Thus  the  myotome  is  subdivided  into 
layers  and  groups  of  cells  which  shift  about 
in  various  directions  to  become  the  skeletal 
muscles  of  the  adult.  The  mesenchyma  around 
them  forms  fascia  and  tendon,  and  connects 
with  the  periosteum  which  is  often  derived  from 
the  sclerotome.  In  the  adult  some  of  the  myo- 
tomes remain  quite  clearly  defined;  thus  the 
muscles  of  each  intercostal  space  are  derived 
from  a  single  mesodermic  segment,  the  ribs 
having  developed  between  them.  In  the  ab- 
dominal muscles  several  segments  have  fused. 
The  muscles  of  the  limbs  are  supposed  to 
arise  from  myoblasts  which  have  migrated  into 
them  from  the  myotomes  of  the  adjacent  body  wall.  Apparently  they 
come  directly  from  mesenchyma.  All  the  striated  skeletal  muscles,  how- 
ever, are  believed  to  come  directly  or  indirectly  from  the  epithehum  of  the 
mesodermic  segments. 

In  cross  section  the  myoblasts  are  of  rounded  outhne  (Fig.  102), 
bounded  by  a  delicate  cell  membrane  or  sarcolemma.  This  membrane 
is  in  close  relation  with  processes  from  the  adjacent  mesenchymal  cells 
and  it  has  been  said  that  the  well  defined  sarcolemma  of  the  adult  is 
essentially  a  product  of  such  cells.  The  myoblasts  consist  of  granular 
protoplasm  (sarcoplasm)  with  coarse  fibrils  near  the  periphery  and  nuclei 
in  the  central  part.     In  a  given  cross  section  the  nuclei  of  many  of  the 


of 


Fig.    ioi.  —  Three    Mksodermic 
Segments    from    Amphibian 
(SiREDON)  Embryos,  of  Suc- 
cessively Older  Stages. 
(Diagrams  after  Mauier.) 

m.,  Muscle  plate;  c.  cutis  plate; 
the  former  is  resolved  into  mus- 
cle fibers,  m.  f.,  the  latter  in 
part  into  muscle  fibers  and  in 
part     into    mesenchyma,    mes. 


STRIATED    MUSCLE. 


87 


myoblasts  will  not  be  included.  In  becoming  muscle  fibers  the  myoblasts 
increase  to  a  diameter  of  from  10  to  100  //-.  The  fibrils  multiply  by  longi- 
tudinal splitting  so  as  to  form  groups  of  fibrils,  or  muscle  columns,  which 
in  cross  section  are  called  Cohnheim's  areas.  Fig.  103  shows  four  adult 
muscle  fibers  cut  across,  in  all  of  which  Cohnheim's  areas  are  distinct. 
Often  such  areas  are  not  distinguishable,  however,  and  when  present  they 
may  appear  as  though  due  to  shrinkage.  Between  the  areas  is  the  sarco- 
plasm  which  may  show  "interstitial  granules"  of  fat  or  lecithin.  The 
nuclei  of  striated  muscle  fibers,  not  seen  in  the  figure,  are  usually  flattened 
and  close  to  the  sarcolemma.  The  fibers  just  described  belong  to  the 
pale  or  white  type.  In  the  dark  or  red  form  the  protoplasm  is  more 
abundant  and  granular,  the  diameter  is  less,  the  fibrils  fewer,  and  the 
nuclei  may  be  central  or  imbedded  among  the  fibrils.     Clearly  this  type 


Connective  tissue. 

Fig.  103. — Cross  Skction  of  Four  Muscle  Fibers 

OF   THE    Hu.MAN   VoC.-\L    MUSCLE.        X  590. 


Fig.  102.— Cross  Section  of  Myoblasts 

AND   Mesenchymal  Cells  fro.m  an 

i8  MM.  Pig. 
mes.,  Mesenchymal  cell ;  f.,  fibril ;    n.,   nu- 
cleus ;  s.,  sarcolemma,  of  a  myoblast. 

is  intermediate  between  the  myoblast  and  the  pale  form.  The  dark  fibers 
contract  more  slowly  than  the  Kght  ones,  but  are  less  easily  fatigued. 
They  are  found  in  the  ocular  muscles  and  in  those  of  mastication  and  of 
respiration.  In  some  single  muscles  both  types  with  intermediate  forms 
may  be  observed.     Ordinarily  striated  muscle  is  of  the  pale  t}TDe. 

The  mesenchyma  surrounding  the  myoblasts  becomes  connective 
tissue.  It  envelops  each  fiber  as  shown  in  Fig.  103,  and  in  progressively 
wider  bands  it  §urrounds  small  bundles  of  fibers,  large  groups  of  these 
bundles,  and  the  entire  muscle  as  shown  in  Fig.  104.  The  connective 
tissue  layer  which  covers  the  whole  muscle  is  the  external  perimysium; 
its  prolongations  into  the  muscle  form  the  internal  perimysium.  It 
contains  fine  longitudinal  elastic  elements  and  sometimes  fat,  chiefly  in 
the  outer  layer.      Elastic  substance  is  particularly  abundant  in  the  dia- 


HISTOLOGY. 


phragm.     Lymphatic  and  blood  vessels  and  nerves  extend  through  the 
perimysium.     The    lymphatic   vessels   end    before    reaching    its    smaller 


Muscle  spindle. 


Cross  section 
of  nerve. 


Connective  tissue 


Fig.  104. — From  a  Cross  Section  of  the  Omohyoid  Muscle  op-  Man.     X  60. 

subdivisions.  Capillary  blood  vessels  are  found  between  the  individual 
fibers,  with  which  they  tend  to  be  parallel.  The  nerves,  chiefly  motor, 
terminate  on  the  fibers.     Sensory  nerves  are  associated  with  the  muscle 

spindles  (Figs.  104  and  105) 

Muscle  fiber.        Connective  tissue 


which  in  cross  section  are 
small  groups  of  slender 
fibers,  containing  many  nu- 
clei. (For  further  descrip- 
tion see  page  103.) 

Since    adult    striated 
muscle  fibers  attain  a  length 
of  from  50  to  120  millime- 
ters, complpte   longitudinal 
sections  of  them  are  seldom 
seen.  A  single  fiber  contains 
very  many  nuclei  (scores  or  perhaps  hundreds),  generally  flattened  oval 
structures  just  inside  the  sarcolemma.    Sometimes  the  nuclear  membrane 
is  indented  by  the  adjacent  fibrils.     The  sarcolemma  is  most  clearly  seen 


Cross  section     Muscle  filjcrs     Nucleus     Nucleus  of  the 
of  nerve.  of  the  of  the         sarcolemma. 

spindle.      perimysium. 
Fig.  105.— The  Muscle  Spindle  shown  in  Fig.  104.    X  240. 


STRIATED    MUSCLE. 


89 


Fig.  106.— Striatkij 
Fiber  of   Frog, 


-Muscle 
Teased 
Apart  in  Water,  Being 
Torn  at  x,  and  showing 

THE      SaRCOLEMMA      AT      S 
AND    S'. 


—k 


in  fresh  fibers  within  which  the  fibrils  have  been  ruptured  and  have  drawn 
away  from  the  membrane  (Fig.  106).     It  resists  acetic  acid  and  has  been 
considered  elastic.      These  fibers  arise  from  myo- 
blasts  which   at  first    have  single  nuclei  within  [         \ 
their  central  portions.    As  the  cells  elongate  their 
nuclei  divide  rapidly,  at  first  by  mitosis  and  later, 
it  is  said,  by  amitosis.     It  is  generally  denied  that          •^  - 
the  adult  fibers  are  due  to  a  fusion  of  myoblasts. 
The  first  fibrils  are  homogeneous  structures  at  the  x 
periphery  of  the  cells.     It  has  been  observed  that 

the  activity  of  certain  muscles  in  living  embryos  /\ '-        ""  "    ;, 

begins  at  the  time  that  their  fibrils  appear.     As  '".^__^       ::§: 

the  fibrils  multiply  and  fill  the  cell  the  nuclei 
migrate  toward  the  sarcolemma.  The  striations 
which  have  been  described  under  cardiac  muscle, 
are  most  perfectly  developed  in  the  voluntary  mus- 
cles. All  that  can  ordinarily  be  seen  of  them, 
however,  is  shown  in  Fig.  107,  namely,  the  alter- 
nating dark  and  hght  bands,  the  latter  bisected 
by  the  ground  membrane.  Sometimes,  though 
rarely,  as  a  result  of  treatment  with  alcohol  the 
muscle  fiber  breaks  into  transverse  discs,  called 
sarcoiis  elements,  each  having  the  thickness  of  a 
muscle  segment.  These  elements  are  single  layers 
of  cuboidal  blocks,  one  for  every  longitudinal  fibril, 
and  these  blocks  may  separate  from  one  another. 
Neither  the  elements  nor  their  small  pieces  are 
now  considered  significant. 

The  extremities  of  the  muscle  fibers  are 
rounded  or  conical,  the  end  toward  the  tendon 
being  more  obtuse  than  the  other.  Near  the  tendon 
the  fiber  contains  many  nuclei  both  peripheral 
and  deeply  placed.  They  divide  by  amitosis  and 
provide  for  lengthwise  growth  of  the  fiber.  Con- 
nection with  the  tendon  is  established  by  the  peri- 
mysium which  is  continuous  with  the  tissue  of  the 
tendon.  The  sarcolemma  ends  with  the  muscle 
substance.  Such  striated  muscle  fibers  as  are  in- 
serted in  the  skin  or  mucous  membranes  may  be 
pointed  or  branched  (Fig.  108).  Their  perimysium 
is  prolonged  in  the  form  of  elastic  fibers  which 
blend  with  the  surrounding  connective  tissue. 


Fig.  107. — Part  of  a  Longi- 
TLDiiNjiL  View  of  a  Hu- 
man Striated  Muscle 
Fiber. 

a.,  Anisotropic;  i.,  isotropic 
band;  k.,  nucleus;  q., 
ground  membrane  fZ). 
X  .560. 


Fig.  ioS.  —  Branched  Stri- 
ated Muscle  Fiber  fro.m 
THE  Tongue  of  .■\  Frog. 


go  HISTOLOGY. 

The  diameter  of  muscle  fibers  is  greater  in  large  animals  than  in 
small  ones;  it  is  increased  by  functional  activity;  and  varies  with  the 
general  nutrition  so  that  the  caliber  may  become  perhaps  trebled.  It  is 
doubtful,  however,  if  any  new  striated  muscle  fibers  develop  in  the  adult. 
Some  have  said  that  they  are  constantly  being  worn  out  and  that  new  ones 
form  to  take  their  places,  developing  from  latent  myoblasts.  It  seems 
to  be  generally  considered  that  the  formation  of  new  fibers  ceases  in  the 
embryo;  muscle  destroyed  by  injury  is  not  restored  in  the  higher  animals. 
The  origin  of  muscle  fibers  by  division  of  those  already  formed,  rather 
than  by  the  development  from  myoblasts,  is  also  generally  denied. 

Striated  muscle  occurs  not  only  in  the  muscles  of  the  limbs  and  body 
wall,  but  also  in  the  ocular  and  ear  muscles,  the  diaphragm,  the  tongue, 
pharynx,  larynx  and  upper  half  of  the  oesophagus,  and  in  parts  of  the 
rectum  and  genital  organs. 


NERVE  TISSUE. 

Irritability  and  conductivity  have  already  been  mentioned  as  funda- 
mental properties  of  protoplasm.  Response  to  particular  irritants  be- 
comes the  chief  function  of  certain  cells.  Thus  some  cells  in  the  eye  are 
differentiated  to  react  to  light;  some  in  the  ear  respond  to  oound;  the 
taste  cells  of  the  tongue  and  olfactory  cells  in  the  nose  are  affected  by 
solutions;  tactile  cells  are  influenced  by  pressure,  and  muscle  cells  contract 
at  the  stimulus  of  the  nervous  impulse.  The  effects  of  irritation  may 
be  conveyed  from  one  part  of  the  cell  to  another  through  its  power  of 
conduction.  Thus  wllen  a  muscle  fiber  is  stimulated  at  one  point,  a  wave 
of  contraction  may  be  transmitted  along  its  whole  extent;  or  when  an 
olfactory  cell  is  stimulated,  the  effects  may  be  conveyed  through  a  long 
fiber-like  basal  prolongation  toward  the  brain.  For  the  purpose  of 
connecting  these  particularly  irritable  cells  there  exists  a  specially  modified 
median  longitudinal  tract  of  ectoderm,  the  nervous  system.  Some  of  its 
cells  send  out  slender  prolongations,  know  as  nerve  fibers,  to  meet  the  taste 
cells,  the  auditory  cells,  the  processes  of  the  nasal  cells,  the  cells  of  the 
muscle  spindles  or  the  epithelial  cells  of  the  skin,  and  to  branch  in  contact 
with  them.  The  effects  of  stimulating  the  various  irritable  cells  enumer- 
ated, are  conducted  along  these  nerve  fibers  back  to  the  central  nervous 
tract.  Such  fibers  as  convey  peripheral  stimuli  to  the  central  system  are 
called  afjerent  or  sensory  fibers;  they  are  the  outgrowths  of  sensory  cells. 
Another  set  of  nerve  fibers  grows  out  from  the  central  tract  and  branches 
in  contact  with  muscle  cells,  smooth  or  striated.  Since  they  transmit 
stimuli  which  cause  the  muscles  to  contract  they  are  called  mo/or  fibers,  and 


NERVE    TISSUE.  9 1 

the  cells  of  which  they  are  a  part  are  the  motor  cells.  The  efferent  fibers, 
or  those  which  bear  impulses  from  the  central  tract  to  the  periphery,  in- 
clude the  motor  fibers,  and  also  some  which  pass  to  the  epithelium  of  glands 
to  control  their  activity.  Besides  the  afferent  sensory  and  the  efferent 
motor  fibers  there  is  a  third  set  of  commissural  cells  and  fibers,  serving  to 
connect  the  other  two.  Sensory  and  motor  cells  may  connect  without 
the  intervention  of  commissural  cells,  thus  providing  a  path  for  the  simplest 
form  of  unconscious  reflex  action,  but  often  one  or  more  commissural 
cells  are  interposed  and  the  brain  consists  essentially  of  these  cells.  As 
the  nervous  impulse  is  transferred  from  cell  to  cell,  being  further  removed 
from  the  primary  stimulus,  it  is  suggested  that  it  becomes  "more  sub- 
jective and  personal." 

The  nervous  system,  then,  is  a  median  longitudinal  tract  of  ectodermal 
cells,  divisible  into  afferent  (sensory),  efferent  (motor),  and  commissural 
cells.  The  sensory  and  motor  cells  send  out  processes  or  fibers,  which 
in  bundles  called  nerves  extend  through  the  mesenchymal  tissue  to  all 
parts  of  the  body.  The  central  tract  is  called  the  central  nervous  system 
and  consists  of  the  brain  and  spinal  cord.  The  nerves  constitute  the 
peripheral  nervous  system.  Associated  with  the  nerves  there  are  clumps 
of  nucleated  bodies  of  nerve  cells,  known  as  ganglia.  The  afferent  and 
efferent  fibers  to  the  viscera  and  blood  vessels,  together  with  numerous 
ganglia,  constitute  the  sympathetic  nervous  system.  The  nervous  system, 
therefore,  is  composed  of  central,  peripheral,  and  sympathetic  portions. 

Development  of  Nerve  Tissue. 

The  Central  Tract.  The  ectoderm  in  an  early  stage  forms  a  flat 
layer  covering  the  embryo  (Fig.  109  A).  Along  the  axial  line  and  extend- 
ing on  either  side  of  it,  the  ectoderm  thickens  to  form  the  medullary  plate. 
The  plate  becomes  depressed  so  as  to  make  a  longitudinal  groove,  the 
medullary  groove  [or  neural  groove]  (Fig.  109  B).  The  dorsal  edges  of 
the  groove  come  together  and  fuse,  transforming  it  into  the  medullary  [or 
neural]  tube  (Fig.  109  C).  Thus  the  tube  becomes  separated  from  the 
general  layer  of  ectoderm  which  is  to  form  the  epidermis.  This  medullary 
tube  is  the  central  nervous  system.  In  its  anterior  part  the  cavity  is  trans- 
formed into  a  series  of  connected  dilated  spaces  or  ventricles,  and  its 
walls  become  very  thick,  thus  forming  the  brain.  The  posterior  part 
makes  the  spinal  cord;  its  walls  are  less  extensively  but  more  uniformly 
thickened  than  those  of  the  brain,  and  its  cavity  remains  small,  becoming 
the  central  canal.  This  canal  is  continuous  with  the  ventricles  of  the 
brain  and  a  line  of  division  between  the  spinal  cord  and  brain  must  be 


92 


HISTOLOGY. 


arbitrarily  drawn.     The  relations  of  the  medullary  tube  to  other  structures 
in  the  embryo  have  been  shown  in  Figs.  19-21,  p.  19-22. 


o.b. 


dr^/ 


spg- 


7  '/T^v 

:,';,,     K-—VT     ep'    g.V  wl 
dra  I-;,'   «y  ^      *= 


v.ra— /;'  'A,*,     ^.^ 

Fig.  109. — The  Development  ok  the  Nervous  System  as  seen  in  Cross  Sections  of  Rabbit 
Embryos:   A,  7^2  Days;  B,S'A  Days;  C,9  Days;  D,  10^2   Days;  E,  14  Days. 
C.  C,  Central  cavity  ;  d.  r.,  dorsal  root ;  d.  ra.,  dorsal  ramus  ;  ep..  ependymal  layer;  g.  c.,  ganglion  cells  ; 
g.  I.,  gray  layer;  m.  g.,  medullary  groove  ;  m.  t.,  medullary  lube  ;  0.  b.,  oval  bundle  ;   S.  g.,  sympathetic 
ganglion  ;  sp.  g.,  spinal  ganglion  ;  s.  ra.,  sympathetic  ramus  ;  v.  r.,  ventral  root ;  v.  ra.,  ventral  ramus  ; 
w.  I.,  white  layer. 

The  Spinal  Ganglia.  At  about  the  time  when  the  medullary  tube 
separates  from  the  epidermal  ectoderm,  some  cells  which  are  detached 
from  its  median  dorsal  portion  pass  down  on  either  side  of  the  tube,  as 
shown  in  Fig.  109  C  and  D.  Through  mitotic  division  these  cells  ac- 
cumulate in  paired  masses  corresponding  in  number  with  the  segments 
of  the  body.     They  are  the  spinal  ganglia.     A  typical  cell  of  a   spinal 

ganglion  is  at  first  round,  but  later  becomes 
bipolar  by  sending  out  two  processes,  one 
toward  the  periphery  and  the  other  toward 
the  medullary  tube.  These  processes  grow 
out  from  opposite  sides  of  the  cell  (Fig.  no). 
With  further  growth  the  nucleated  cell  body 
passes  to  one  side  of  its  prolongations,  with 
which  it  remains  connected  by  a  slender  stalk. 
These  T-shaped  cells  are  characteristic  of 
the  spinal  ganglia.  The  fibers  which  grow  toward  the  medullary  tube 
enter  its  outer  part  and  fork,  sending  one  branch  tow3.rd  the  brain  and  the 
other  down  the  cord.  There  are  many  of  these  j^arallel  fibers  extending 
toward  the  brain  .so  that  they  form  distinct  bundles,  one  on  either  side  of 
the  cord,  known  as  oval  bundles  (Fig.  109,  E).     Since  they  receive  acces- 


Dipolar  cells 


Fig.  no. — Spinal  Ganglion  Cells. 
THE  RipoLAK  Forms  from  a  6 
Day  Chick  Embryo. 


NERVE    TISSUE. 


93 


sions  of  fibers  from  every  spinal  ganglion,  they  enlarge  as  they  approach 
the  brain.  The  fibers  of  the  oval  bundle  branch  freely  at  their  termination 
and  also  give  off  collateral  branches  along  their  course,  which  enter  the 
deep  substance  of  the  cord.  The  peripheral  fibers  from  the  spinal  gangha 
elongate  through  the  mesenchyma,  and  terminate  in  branches  applied  to 
cells  in  the  skin  or  muscle  spindles,  in  ways  to  be  described  presently. 
The  fibers  of  the  spinal  ganglia  are  essentially  afferent  or  sensory,  and  they 
proceed  from  sensory  cells. 

The  Ventral  Roots.  The  efferent,  motor  fibers  arise  chiefly  from 
cells,  the  bodies  of  which  remain  within  the  central  nervous  system.  Each 
of  these  cells  sends  out  one  long  process  called  a  neuraxon  (axone).  The 
neuraxons  of  the  motor  cells  leave  the  spinal  cord,  near  its  ventral  surface, 
in  bundles  which  are 
segmentally  arranged 
so  that  they  corre- 
spond with  the  spinal 
ganglia.  A  bimdle  of 
motor  fibers  joins  a 
bundle  of  peripheral 
fibers  from  a  spinal 
ganglion  to  form  a 
spinal  nerve.  Every 
spinal  nerve  conse- 
quently has  a  dorsal 
(sensory)  root,  and  a 
ventral  (motor)  root. 
The  fibers  from  the 
two  roots  travel  in  the 

same  connective  tissue  sheath,  but  otherwise  they  remain  entirely  distinct. 
The  motor  fibers  terminate  in  contact  with  muscle  cells.  Soon  after  a 
spinal  nerve  is  formed  by  the  junction  of  its  roots,  it  divides  into  a  dorsal 
ramus  and  a  ventral  ramus  (Fig.  109,  E).  These  rami  are  mixed  nerves 
(containing  both  sensory  and  motor  fibers)  and  supply  the  skin  and  muscles 
of  the  back  and  of  the  lateral  body  wall  respectively. 

Within  the  cord  the  motor  cells  send  out  a  large  number  of  com- 
paratively short  branching  processes  called  dendrites.  By  means  of  the 
dendrites  the  motor  cell  is  put  in  communication  with  the  collateral  fibers 
of  the  sensory  cells,  and  with  fibers  of  commissural  cells  coming  either 
from  other  parts  of  the  cord  or  from  the  brain.  This  arrangement  is 
shown  in  the  diagram  Fig.  in.  A  painful  stimulus  transmitted  along  the 
sensory  fiber,  h,  passes  through  the  spinal  ganghon  into  the  cord.     Through 


Fig.  III. 
Diagram  of  the  spinal  cord,  showing  a  motor  fiber,  a;  a  sensory  fiber, 
b  and  c  ;  and  a  commissural  fiber,  d,  from  the  brain  ;  coll.,  collateral 
fiber;  sp.  g.,  spinal  ganglion. 


94. 


HISTOLOGY 


collateral  branches  it  may  be  transmitted  to  the  motor  fiber,  a,  causing  a 
muscle  to  contract  involuntarily.  This  is  the  reflex  path.  Or  the  stimulus 
from  b  may  be  conveyed  to  the  brain  along  the  fiber  c,  and  be  transferred 
to  commissural  cells  of  which  ^  is  a  fiber  extending  down  the  cord.  This 
also  may  stimulate  the  motor  cell  a,  causing  the  muscle  to  contract  volun- 
tarily. 

The  terms  dendrite  and  neuraxon  are  of  wide  application.  A  nerve 
cell  generally  has  a  single  process  which  differs  from  the  others  in  being 
clear,  non-granular,  and  sharply  defined,  often  becoming  very  slender 
soon  after  leaving  the  cell  body.  It  may  have  collateral  branches,  usually 
given  off  at  right  angles,  but  except  at  its  termination  its  branches  are 
relatively  few.  It  conducts  impulses  away  from  the  cell  body.  This  process 
is  the  neuraxon.  The  dendrites,  which  develop  later,  appear  as  granular, 
protoplasmic  processes.  They  fork  and  branch  freely,  giving  the  cell  a 
great  extent  of  exposed  surface.  They  may  serve  in  obtaining  nutriment, 
as  well  as  in  providing  many  opportunities  for  contact  with  the  processes 
of  other  nerve  cells.-  Dendrites  conduct  impulses  toward  the  cell  body. 
In  the  sensory  cells  of  the  dorsal  ganglion  the  single  peripheral  fiber  is  a 
dendrite  of  unusual  form,  and  the  fiber  entering  the  cord  is  the  neuraxon. 

The  Sympathetic  System  develops  chiefly  from  the  visceral  or 
sympathetic  branches  of  the  spinal  nerves.  A  spinal  nerve  typically 
has  one  such  branch,  extending  ventrally  and  medially  toward  the  aorta, 
and  ending  in  a  clump  of  nerve  cells  (Fig.  109  E).  These  cells,  which 
constitute  a  sympathetic  ganghon,  are  considered  to  have  migrated  along 
the  nerve  bundles  from  the  spinal  ganghon,  or  possibly  from  the  spinal 
cord:  They  multiply  by  mitosis.  The  successive  gangha  become  con- 
nected by  longitudinal  nerve  fibers  so  that  they  form  two  sympathetic 
trunks  (or  cords),  one  on  either  side  of  the  vertebral  colunm.  The  gangha 
of  the  sympathetic  trunk  are  cervical,  thoracic,  lumbar  and  sacral.  There 
are  only  three  cervical  gangha,  probably  because  some  in  this  region  have 
fused.  In  the  adult  the  sympathetic  gangha  are  each  usually  connected 
with  the  spinal  nerves  by  two  bundles  of  fibers,  the  white  and  gray  rami 
respectively.  The  smaller  gray  ramus  is  said  to  convey  fibers  from  the 
ganglion  to  the  spinal  nerve.  These  rami  may  be  subdivisions  of  the  orig- 
inal visceral  branch. 

Besides  smaller  branches  from  the  three  cervical  gangha  to  neighbor- 
ing vessels  and  organs,  each  of  these  ganglia  sends  out  a  large  cardiac 
nerve,  the  branches  of  which  unite  to  form  the  cardiac  plexus.  From 
this  plexTis  and  the  associated  cardiac  ganglion  the  fibers  continue  to  the 
heart  muscle  which  they  innervate.  In  the  lower  thoracic  region  the 
ganglia  of  the  sympathetic  trunk  send  out  nerve  bundles  which  unite  to 


SYMPATHETIC    NERVES. 


95 


form  the  splanchnic  nerves.  These  pass  along  the  sides  of  the  aorta,  in 
front  of  which  they  form  a  large  plexus,  the  coeliac  [or  solar]  plexus,  as- 
sociated with  which  is  the  coeliac  [or  semilunar]  ganghon  (Fig.  112). 
A  plexus  is  a  net  of  nerves  which  allows  a  transfer  of  fibers  from  one 
bundle  to  another;  the  individual  nerve  fibers  probably  do  not  anastomose. 
In  the  sympathetic  plexuses  there  are  usually  nerve  cells,  called  ganghon 
cells,  often  found  at  the  angles  of  the  network.  In  contact  with  them 
the  nerve  fibers  may  terminate.  WTien  these  cells  are  very  abundant 
the  plexus  becomes  a  ganghon.  From  the  coehac  ganghon,  fibers  pass 
into  the  intestine  and  form  a  ganglionated  plexus  between  the  muscle 
layers,  called  the  myenteric  plexus.  Branches  from  it  innervate  the 
muscles  and  pass  on  to  make  another  plexus  under  the  intestinal  epithehum, 
the  submucous  plexus.  Finally  they  come  very 
close  to  the  epithelium  itself. 

All  of  the  nerve  cells  of  the  sympathetic 
system  are  beheved  to  be  ectodermal,  and  de- 
scendants of  those  which  migrated  from  the 
spinal  gangha  or  central  nervous  system.  All 
the  sympathetic  nerve  fibers  are  processes  of 
such  cells,  and  they  are  found  forming  plexuses 
around  the  blood  vessels  and  organs,  including 
those  of  the  intestinal  tract,  the  bladder,  kid- 
ney, suprarenal  gland  and  spleen.  Two  fea- 
tures of  the  sympathetic  system  seem  funda- 
mental ;  their  fibers  supply  the  viscera,  and  they 
are  so  connected  with  peripheral  ganghon  ceUs 
that  they  act  more  or  less  independently  of  the 
central  nervous  system. 

The  Cerebeal  Nerves.  The  nerves  con- 
nected with  the  brain  are  not  a  series  of.  similar  structures  hke  the  spinal 
nerves.  Four  of  them  possess  only  ventral  motor  roots.  Four  others  have 
dorsal  sensory  roots  provided  with  gangha,  and  lateral  motor  roots.  Lateral 
roots  emerge  just  ventral  to,  or  beneath  the  dorsal  roots.  Their  fibers  are 
the  neuraxons  of  ceUs,  the  bodies  of  which  remain  within  the  central  ner- 
vous system.  Lateral  root  fibers  occur  as  far  down  the  cord  as  the  sixth 
cervical  ganglion.  Instead  of  entering  the  corresponding  cervical  nerves, 
however,  these  fibers  unite  to  form  a  bundle  which  passes  along  just  outside 
of  the  spinal  cord,  through  the  foramen  magnum  into  the  skull  where  it 
becomes  the  accessory  portion  of  the  vagus  nerve.  Below  the  sixth  cer\dcal 
ganghon  the  lateral  root  elements  have  not  been  demonstrated.  (It  has 
been  suggested  that  they  pass  out  in  the  dorsal  roots,  and  that  they  form 
parts  of  the  ventral  roots.) 


Fig.  112. 
Diagram  of  the  sympathetic  system 
in  its  relation  to  the  intestine, 
int.;  A.,  aorta;  sp.  g.,  spinal 
ganglion  ;  w.  r.,  white  ramus  ; 
g.  t.,  ganglion  of  the  sympa- 
thetic trunk;  spl.,  splanchnic 
ner\'e;  coe.  g.,  coeliac  ganglion  ; 
my.  pi.,  myenteric  plexus;  sbm. 
pi.,  submucous  plexus. 


q6  .  HISTOLOGY. 

In  the  diagram  Fig.  113,  based  upon  the  nerves  in  a  12  mm.  pig  embryo, 
the  roots,  ganglia,  and  fundamental  branches  of  the  cerebral  nerves  are 
indicated.  The  ventral  roots  have  been  shaded  by  lines.  The  hypoglossal, 
abducens,  trochlear  and  oculomotor  nerves  are  ventral  roots  only,  the  first 
going  to  muscles  of  the  tongue  and  throat,  the  other  three  supplying 
muscles  of  the  eye.  The  trochlear  nerve  is  unique  in  having  its  neuraxons 
pass  to  the  upper  eide  of  the  brain  and  cross  to  the  opposite  side  before 
emerging.  Four  cerebral  nerves  are  mixed,  consisting  of  dorsal  and  lateral 
roots.     Beginning  posteriorly  these  are  the  vagus  (its  motor  part  being 


Fig.  113. — The  Ckrebral  Nervks  of  a  12  mm.  Pig,  Named  in  the  Order  of  their  Occurrence 
Beginning  Anteriorly,  with  their  Ganglia  and  Chief  Branches. 

Olfaclnry  (not  developed).  Optic  (fibers  in  the  stalk  of  the  eye,  tlie  lens  of  which  is  marked  L).  Oculo- 
motor (Oc).  Trochlear  (Tr.).  Trigeminal. — semilunar  ganglion  (s.-l.l;  ophthalmic  (oph.V  maxil- 
lary (mx.)  and  mandibular  (md.i  branches..  Abducens  lAb.).  Ititernifdiua, — geniculate  ganglion  (g.); 
large  superficial  petrosal  (I.  s.  p.).  chorda  tympani  (ch.ty.),and  facial  'faj  branches.  Acoustic  (tiS> 
its  ganglion  being  later  divided  into  a  vestibular  ganglion,  and  a  spiral  ganglion.  It  supplies  the 
otocyst  (Ot.i.  Glossopharyngeal, — superior  (s.l  and  jjetrosal  (p.)  ganglia;  tympanic  (t.K  lingual 
(I.  r.)  and  pharyngeal  (ph.  r.)  branches.  I'agus. — jugular  (j.)  and  nodose  (n.l  ganglia  ;  auricular  (au.) 
and  laryngeal  branches,  rec.  being  the  recurrent  nerve;  the  main  stem  proceeds  to  the  stomach.  Its 
accessory  portion  has  an  external  ramus  (ex.).  Hypoglossal  (Hy. ).  Froriep's  rudimentary  hypo- 
glossal ganglion  (F.)  sometimes  sends  fibers  to  the  hypoglossal  nerve,     c.1,  C.2,  c.3,  cervical  nerves. 


called  the  accessory  nerve),  the  glossopharyngeal,  the  intermedius  (its  motor 
part  and  its  largest  branch  forming  the  facial  nerve),  and  the  trigeminus. 
In  the  diagram  the  lateral  roots  are  in  solid  black  and  the  dorsal  roots  are 
not  shaded.  The  accessory  nerve  is  seen  passing  up  the  spinal  cord  to 
join  the  vagus.  A  part  of  its  fibers  turn  aside  in  the  external  ramus,  ex, 
to  supply  the  trapezius  and  sterno-cleido-mastoid  muscles;  others  remain 
with  the  vagus  to  supply  pharyngeal  muscles,  and  to  pass  down  the  body 
to  the  stomach.     The  vagus  and  theglossopharyngeus  each  have  two  ganglia, 


CEREBRAL   NERVES.  97 

one  above  the  other.  The  lower  gangha  occur  near  the  epidermis  of  the 
embryo  in  positions  said  to  correspond  with  the  epibranchial  sense  organs 
of  fishes.  These  organs  do  not  develop  in  man,  but  the  gangha  are 
permanent  structures.  Closely  united  with  the  geniculate  ganglion  of 
the  intermedins  is  the  ganghon  of  the  acoustic  nerve.  The  latter  is  a 
purely  sensory  nerve  to  the  ear.  By  some  comparative  anatomists  it  is 
considered  a  part  of  the  intermedins.  In  the  trigeminus  it  is  to  be  noted 
that  the  lateral  root  joins  the  mandibular  division  only.  The  pecuhar 
optic  and  olfactory  ner\^es  will  be  considered  with  the  sense  organs. 

The  sympathetic  system  in  the  head  supphes  the  smooth  muscles  of 
the  blood  vessels  and  iris,  together  with  parts  of  the  pharyngeal  mucous 
membranes  and  the  saHvary  glands;  it  sends  fibers  into  the  periosteum. 
The  plexuses  around  the  large  blood  vessels  are  continuous  with  the 
sympathetic  plexuses  of  the  neck.  Although  the  cerebral  nerves  do  hot 
have  any  regularly  arranged  sympathetic  or  visceral  rami,  all  of  them, 
except  the  olfactory,  optic,  and  acoustic,  are  said  to  communicate  with 
the  sympathetic  system.  In  the  head  there  are  four  sympathetic  gangha, 
the  ciliary  J  sphenopalathie,  otic  and  submaxillary,  ah  of  which  are  connected 
with  the  trigeminal  nerve.  They  develop  later  than  the  semilunar  ganghon 
from  which  their  cells  may  migrate.  The  sphenopalatine,  otic,  and  sub- 
maxillary gangha  are  also  connected  with  the  intermedins  and  may  receive 
cells  from  the  geniculate  ganghon.  The  otic  further  receives  the  continua- 
tion of  the  tympanic  branch  of  the  glossopharyngeus. 

Structure  of  Nerve  Tissue. 

In  the  foUowing  sections  the  structure  of  nerve  fibers  and  of  nerv-es 
win  be  considered  first;  then  the  sensory  and  the  motor  endings;  next 
the  gangha,  spinal  and  sympathetic;  and  finaUy  the  spinal  cord  as  illus- 
trating the  tissue  of  the  central  nervous  system. 

Nerve  Fibers.  The  peripheral  processes  of  nerve  cells  generally 
appear  as  slender  homogeneous  strands  varying  in  diameter.  The  smallest 
are  found  in  connection  with  the  sympathetic  system  and  near  the  termina- 
tions of  the  spinal  nerves;  the  largest  fibers  .are  the  portions  near  the  cord 
of  those  which  have  the  longest  course.  There  is  no  characteristic  differ- 
ence in  diameter  between  sensory  and  motor  fibers. 

With  special  methods  it  has  been  clearly  shown  that  the  nerve  fiber 
consists  of  longitudinal  fibrils  imbedded  in  a  protoplasmic  neuroplasm. 
The  fibrils  begin  in  the  cell  body.  At  the  origin  of  the  neuraxon  they  may 
appear  as  if  gathered  into  one  coarse  sthf  fibril  which  distally  is  resolved 
into  a  bundle.  The  fibrils  are  supposed  to  divide  but  presumably  they 
do  not  form  networks.  When  the  fiber  branches  the  fibrils  separate  into 
7 


98 


HISTOLOGY. 


Fig.  114.- 


-NON-MEDUI.LATED    NERVE    FIBERS. 

(After  Schafer.) 


X  400. 


corresponding  groups.  They  are  considered  to  be  the  essential  conducting 
element  of  nerves,  but  it  is  known  that  conduction  occurs  in  protoplasm 
in  which  librils  cannot  be  demonstrated. 

As  the  fibers  in  the  embryo  grow  out  from  the  central  nervous  system 
they  form  bundles,  in  and  around  which  there  are  numerous  nuclei. 
Opinions  differ  as  to  whether  these  nuclei  belong  with  the  mesenchymal 

cells  through  the  meshes  of 
which  the  nerve  is  growing, 
or  with  ectodermal  cells  car- 
ried along  from  the  spinal 
gangha  or  cord.  In  either 
case  they  are  called  sheath  cells,  and  are  "so  closely  apphed  to  the  fibers 
that  it  becomes  a  matter  of  judgment  to  decide  whether  the  fibrils  are  sur- 
rounded by  or  imbedded  in  the  sheath  cells."  Therefore  some  writers 
have  thought  that  the  nerve  fiber  was  not  the  outgrowth  of  a  single  cell, 
but  was  produced  by  the  end  to  end  anastomosis  of  many  sheath  cells, 
each  of  which  formed  that  portion  of  the  nerve  fiber  which  it  enclosed. 
Since  the  fiber  may  be  a  meter  long  ^  ,     , 

and  perhaps  ten  thousand  times  the 
diameter  of  the  cell  body  from  which 
it  comes,  such  an  assumption  seems 
plausible;  nevertheless  it  is  not  sus- 
tained by  recent  embryological  in- 
vestigations. 

The  cells  applied  to  the  nerve 
fiber  may  unite  and  thus  surround  it 
with  a  delicate  homogeneous  sheath 
called  the  neurolemma  [sheath  of 
Schwann].  Some  fibers  in  the  adult, 
especially  in  the  sympathetic  system, 
possess  only  a  sheath  of  this  sort, 
and  they  are  called  non-medullatcd 
fibers  (Fig.  1 14).  Other  fibers  in  the 
sympathetic  system  and  near  the 
nerve  terminations  may  be  sur- 
rounded only  by  ordinary  connective  tissue;  these  are  non-medullated  fibers 
without  a  neurolemma  [naked  axis  cylinders].  (Non-medullated  fibers  of 
the  sympathetic  system  are  often  called  Remak's  fibers.)  The  fibers  of  the 
spinal  nerves  are  generally  characterized  by  a  deposit  of  myelin,  found 
between  them  and  the  neurolemma.  The  fibers  with  a  myelin  sheath  are 
called  medullated,  and  the  fibers  themselves  within  the  myelin  sheath, 
whether  they  are  dendrites  or  neuraxons,  arc  called  axis  cylinders. 


J 

nu    C 


•5V    1 


-A  J 


my 


m> 


G 


H 


Fig.  115. — Medullated  Nerve  Fibers. 
A-D,  Longitudinal  .sections;  E-l,  cross  sections.  (A, 
B,  after  Gedoelst;  C,  E,  F,  after  Hardesty;  D 
and  I,  osmic  aciti  preparations,  after  Prenant 
and  ScymonowicK ;  G,  alcoholic  preservation, 
after  Koelliker ;  H,  picric  acid  preservation, 
alter  Schafer.)  a.  C,  Axis  cylinder;  in.,  incis- 
ure; my.,  myelin;  nu.,  nucleus  of  the  neuro- 
lemma. 


NERVE    FIBERS.  99 

Myelin  is  a  mixture  of  complex  fats  and  lipoid  substances,  some  of 
which  are  combined  with  sugar.  Like  fat  it  is  dissolved  by  ether  and 
blackens  with  osmic  acid.  It  exists  as  an  emulsion,  and  appears  very 
white  macroscopically.  Betw^een  the  myelin  globules  there  is  a  network 
of  neurokeratin,  a  substance  unstained  by  osmic  acid  and  not  dissolved 
by  ether.  Fig.  115,  A  and  B,  show  the  neurokeratin  network  after  treat- 
ment with  ether,  surrounding  the  axis  cylinder,  ax.  The  meshes  vary 
greatly  in  diameter,  becoming  coarse  with  the  rapid  post  mortem  coales- 
cence of  myehn  droplets.  Fig.  115,  C,  shows  a  heavier  framework  which 
toward  the  right  of  the  figure  tends  to  form  conical  layers,  the  axis  cylinder 
penetrating  their  apices;  in  E  a  cross  section  of  C  is  drawn  showing  a  myelin 
vacuole,  my,  encircling  the  fiber.  In  specimens  stained  with  osmic  acid 
(D),  the  myelin  is  very  dark  and  the  framework  light.  The  latter  is 
prominent  only  in  oblique  lines  called  in- 
cisures [or  Lantermann's  segments].  The 
lines  seen  on  the  opposite  sides  of  the 
fiber  are  interpreted  as  optical  sections  of 
a  cone  of  neurokeratin.  A  cross  section 
of  D  through  an  incisure  would  appear 
as  in  I.  Successive  incisures  may  point 
in  opposite  directions.  They  do  not  all 
represent  perfect  cones,  but  in  that  form 
they  are  characteristic  post  mortem  fig-  no'        D 

UreS.       rig.   115,  I,  G,  and  H,  show  other      a.  Diagram  of  the  intracellular  explanation 

of  myelin;    B.  the   cross   obtained  with 

cross  sections  of  medullated  fibers  m  which  silver  nitrate,-  C.  the  blconlcal  enlarge- 

ment   (after  GedoelstI;    D,   intercellular 

the  neurokeratin  is  arranged  radially  or         mveiin  (after  Hardesty);  a.  c.axis  cyi- 

"-'  -'  inder ;  ax.,  axolemma  ;   my.,  myelin;  ne.. 

in  concentric  layers.  neurolemma;  no.,  node. 

At  regular  intervals  the  myelin  sheath  is  more  or  less  interrupted  by 
nodes  [of  Ranvier].  The  intervals  vary  from  80  o.  to  a  millimeter,  being 
shorter  in  growing  fibers  and  in  the  distal  portions  of  adult  fibers.  The 
branching  of  medullated  fibers  occurs  at  these  nodes.  Fig.  116,  A,  an 
osmic  acid  preparation,  illustrates  one  interpretation  of  the  myelin  and 
nodes,  according  to  which  the  sheath  cells  are  thought  to  be  wrapped 
around  the  axis  cyHnders,  and  to  contain  within  them  the  myelin  which 
develops  Hke  fat  in  the  mesenchymal  cells.  The  nodes  (A,  no)  are  at 
the  junction  of  two  sheath  cells,  and  there  the  outer  cell  membrane  or 
neurolemma  is  continuous  with  the  axolemma  or  inner  cell  membrane, 
the  latter  being  in  contact  with  the  axis  cylinder.  It  accords  with  this 
view  that  the  neurolemma  usually  has  but  a  single  nucleus,  found  midway 
between  two  nodes.  Surrounded  by  very  httle  protoplasm  it  occupies 
a  depression  in  the  outer  surface  of  the  myelin. 


lOQ 


HISTOLOGY. 


When  nerve  fibers  are  treated  with  a  solution  of  silver  nitrate  a  pre- 
cipitate occurs  at  the  nodes  and  spreads  along  the  axis  cyhnder  forming 
a  cross  (Fig.  ii6,  B).  This  has  been  interpreted  as  indicating  a  penetrable 
intercellular  substance  at  the  nodes  through  which  nutriment  has  access 
to  the  fiber.  Silver  nitrate  sometimes  causes  a  transverse  banding  of  the 
axis  cyhnder,  which  is  considered  artificial  and  without  significance. 
In  crossing  the  node  the  fibrils  may  spread  apart  forming  a  "biconical 
enlargement."  As  shown  in  C,  the  fibrils  in  the  midst  of  the  enlargement 
have  been  said  to  be  thickened.  The  same  figure  suggests  that  the  neuro- 
lemma is  not  continuous  with  an  axolemma  but  passes  the  node  without 
interruption.  This  is  clearly  shown  in  D,  where  the  myehn  layer  also, 
though  constricted,  is  unbroken.  The  myelin  has  therefore  been  regarded 
as  an  exoplasmic  part  of  the  axis  cyhnder.     The  inter-fibrillar  substance 


Fat  cells. 


Artery. 


Bundles  of  nerve  fibers. 


Epineurium. 


Perineurium. 


Endoneurium. 


Fig.  117. — Medullated  Nerve.    Part  of  a  Cross  Section  of  the  Human  Median  Nerve.    X  20. 


of  the  nerve  fiber  has  been  said  to  present  many  characteristics  of  myehn. 
The  close  relation  of  myehn  to  the  cylinder  is  shown  in  ''peripheral  de- 
generation." When  a  nerve  is  severed,  that  portion  of  the  axis  cyhnder 
which  is  cut  off  from  the  cell  body  from  which  it  grew,  degenerates  by 
fragmentation.  The  myehn  at  the  same  time  breaks  up  into  drops  of 
a  different  chemical  nature  which  later  disappear.  The  sheath  cells 
multiply.  Recently  it  has  been  stated  that  the  myehn  should  be  considered 
an  intercellular  substance  due  to  a  transformation  of  tissue  fluid  by  the 
joint  activity  of  the  axis  cyhnder  and  sheath  cells.  It  first  appears  in  the 
embryo  as  vesicles  attached  to  the  nerve  fiber.  These  unite  to  form  a 
nodular  or  beaded  layer  which  later  becomes  smooth.  The  axolemma 
is  considered  a  condensation  of  the  myehn  framework  such  as  occurs  also 
just  beneath  the  neurolemma.  The  myehn  itself  is  said  to  be  derived 
from  thejblood. 


NERVES. 


lOI 


Nerves  are  bundles  of  nerve  fibers  enveloped  in  connective  tissue 
sheaths.  According  to  the  nature  of  their  constitutents  they  are  classed 
as  medullated  and  non-medullated,  a  distinction  which  the  student  should 
remember  to  record. 

The  spinal  and  cerebral  nerves  consist  mostly  of  medullated  fibers 
of  varying  diameter  (2-20  /->■),  scattered  among  vv^hich  are  a  few  that  are 
non-medullated.  Medullated  nerves  are  white  in  reflected  hght.  They 
are  surrounded  by  loose  connective  tissue  [the  epineurium]  which  contains 
lymphatic  and  blood  vessels,  and  small  nerves,  and  has  many  elastic 
fibers.  ,  It  extends  around  the  entire  nerve  and  between  the  several  well 
defined  bundles  of  which  a  large  nerve  consists  (Fig.  117).  Each  of  these 
bundles  is  covered  by  a  dense  lamellar  layer  of  flattened  connective  tissue, 


Fiber  sheath. 


Fig.  118.— Medullated  Nerve.    Part  of  a  Cross  Section  of  the  Human 
Median  Nerve.    X  220. 

called  the  perineurium  (Fig.  ii8).  The  cells  in  the  perineural  layers  are 
in  contact  with  one  another  along  their  borders  so  that  on  surface  view 
they  resemble  a  mesothelium.  The  perineurium  sends  septa  into  the 
nerve  bundle  and  becomes  continuous  with  the  connective  tissue  which, 
outside  of  the  neurolemma,  surrounds  each  individual  nerve  fiber  [Henle's 
sheath].  The  inner  extensions  of  the  perineurium  may  be  called  internal 
perineurium  (or  endoneurium).  The  perineurium  contains  capillaries, 
generally  parallel  with  the  nerve  fibers,  and  tissue  spaces,  but  no  lymphatic 
vessels.  The  outer  sheaths  of  the  nerves  are  continuous  with  the  dura 
mater  of  the  cord  and  brain. 

The  large  sympathetic  nerves  vary  in  color.  The  splanchnic  nerves 
contain  many  medullated  fibers  and  are  whiter  than  the  nerves  of  the 
plexuses.  Medullated  fibers  in  the  latter  are  few  and  very  slender.  Non- 
medullated  nervous  tissue  is  gray.  A  part  of  the  medullated  fibers  of  the 
sympathetic  nerves  come  directly  from  the  spinal  nerves,  and  a  part  are 
medullated   processes    of    the    sympathetic   ganghon    cells.     Small   non- 


I02 


HISTOLOGY. 


•  c.t. 


I^y' 


Fig.  119.— Xdn-mkuillated  Xerves, 
FROM  A  Cat's  Intestine. 

A,  From  the  submucous  and  B,  from 
the  myenteric  plexus,  c.  t..  Con- 
nective tissue;  n.,  sympathetic, 
non-medullated  nerve  fibers;  n.  C. 
nerve  cell ;  s.  m.,  smooth  muscle. 


medullated  nerves  are  shown  in  Fig.  119;  A  represents  a  nerve  which 
is  easily  recognized  by  the  two  large  nerve  cells  which  it  contains;  B 
is  a  bundle  of  fine  fibers  containing  a  few  nuclei,  probably  of  connective 
tissue. 

The  recognition  of  small  nerves  in  ordinary  sections  may  be  facihtated 
by  remembering  that  they  are  fibrous  bundles  extending  through  con- 
nective tissue  and  found  in  the  same  situa- 
tions as  the  vessels.  The  latter  are  tubes 
Hned  with  endothelium.  _  Sometimes  they  are 
filled  with  corpuscles  (Fig.  118)  but  the  cor- 
puscles never  appear  fibrous  and  usually 
stain  unhke  anything  else  in  the  specimen. 
Nerves  differ  in  texture  from  the  W'hite  fiber 
of  connective  tissue,  w^hich  forms  a  diffuse 
network  or  layer  instead  of  occasional  dis- 
tinct circumscribed  bundles. 

Sensory  Endings.  The  way  has  al- 
ready been  described,  in  which  ectodermal 
cells  become  detached  from  the  medullary 
tube  to  form  spinal  and  cerebral  ganglia,  afterwards  becoming  bipolar  and 
then  T-shaped,  sending  a  long  dendrite  through  the  nerve  bundle  to  the 
periphery.  Soon  after  it  leaves  the  cell  body,  this  process  becomes  sur- 
rounded by  the  neurolemma  and  myehn  sheath.  Its  branches  are  very  few 
until  it  nears  its  distal  end  when  it  forks  repeatedly  at  the  nodes.  Finally 
it  loses  its  sheaths  and  is  resolved  into  many  small  fibers  which  terminate 
in  contact  with  epithehal,  connective  tissue  or  muscle  cells.  These  terminal 
branches  of  the  dorsal  root  fibers  are  the  sensory  nerve 
endings.  Apart  from  those  of  the  special  sense  organs, 
to  be  described  with  the  eye,  ear,  etc.,  they  are  as  fol- 
lows. 

Free  nerve  endings.  Sensory  ner\-es  to  the  epi- 
thelia,  such  as  the  epidermis,  or  that  which  forms  part 
of  the  mucous  membrane  of  the  mouth,  or  the  corneal 
epithehum,  lose  their  myehn  sheaths  and  divide  repeat- 
edly in  the  connective  tissue  just  beneath.  The  un- 
sheathed slender  fibers  thus  formed  pass  between  the 
epithelial  cells  where  they  ramify  further,  and  terminate  with  pointed  or 
club-shaped  ends  (Fig.  120).  Such  jree  endings  are  too  delicate  to  be  seen 
in  ordinary  preparations.  Sometimes  the  terminal  fibers  in  the  lower 
layers  of  the  epidermis  expand  into  crescentic  structures  called  tactile 
menisci  (Fig.  121).     An  epidermal  cell,  the  base  of  which  rests  upon  a 


.  120. — Free  Nerve 
Ending,  in  Epi- 
thelium. GOLGI 
Preparation. 
(After  Retzius.) 


SENSORY   ENDINGS.  IO3 

meniscus,  may  thereby  be  modified  so  that  it  is  larger  and  clearer,  having 
a  more  vesicular  nucleus,  than  those  around  it.  Cells  thus  differentiated 
are  called  tactile  cells. 

The  sensory  nerves  to  muscles  similarly  may  end  freely,  or  may  be 
in  special  relation  with  modified  muscle  fibers.  In  the  former  case  (Fig. 
131,  sensory  fibers)  the  nerves  become  non-medullated  and  their  fibers 
arborize  extensively,  terminating  in  long  slender  filaments  between  the 
muscle  cells.  The  specially  modified  muscle  fibers  in  contact  with  which 
sensory  nerves  end,  constitute  the  muscle  spindles  (Fig.  105,  p.  88). 
These  are  bundles  of  from  3  to  20  muscle  fibers,  i  to  4  mm.  long,  varying 
in  width  from  80  to  200  />«.  They  are  surrounded  by  a  thick  connective 
tissue  sheath  or  capsule,  continuous  with  the  perimysium  and  described 
as  divided  into  an  inner  and  an  outer  portion  by  a  considerable  tissue 
space  filled  with  fluid.  The  muscle  fibers  of  the  spindle  are  distinctly 
striated  toward  their  tapering  and  very  slender  ends.     In  their  middle 


Epidermis 


Tactile  cell. 

Tactile 

meniscus.        \ 

Nerve  fiber. 


Connective 

tissue.  x^ 

J 

Fig.  121. — From  a  Vertical  Section  of  the  Skin  of  the   Great  Toe  of  a   Man  Twenty-five 

Years  Old.    X  240. 

The  outlines  of  the  cells  and  the  nuclei  of  the  epidermis  can  only  be  indistinctly  seen,     x,  Tactile  cells 

in  the  corium,  resting  upon  the  ramifications  of  a  delicate  nerve  fiber. 

portions  the  striations  are  obscure;  there  the  sarcoplasm  is  abundant  and 
the  muscle  nuclei  are  numerous.  Three  or  four  nerves  terminate  in  each 
muscle  spindle.  Their  connective  tissue  sheaths  blend  with  the  perimysial 
capsule,  and  they  branch  and  lose  their  myelin  as  they  pass  through  this 
capsule  to  the  muscle  cells.  They  may  encircle  the  muscle  fibers  of  the 
spindle,  forming  spirals  or  rings  (as  in  the  upper  part  of  Fig.  122)  or  they 
may  form  a  panicle  of  branches  with  enlarged  club-shaped  ends.  Muscle 
spindles  are  not  found  in  the  muscles  of  the  eye,  pharynx,  larynx,  and 
oesophagus,  the  muscles  of  expression,  the  diaphragm  and  the  ischio-  and 
bulbo-cavernosus  muscles.  They  are  especially  numerous  in  the  muscles 
of  the  hand  and  foot.  The  nerves  of  the  spindles  are  stimulated  by  pres- 
sure caused  by  the  contraction  of  adjoining  muscle  fibers. 

In  tendons  there  are  said  to  be  free  nerve  endings,  but  the  sensory 
fibers  which  terminate  in  tendon  spindles  are  better  kno^vn.  These 
are  small  portions  of  the  tendon,  from  i  to  3  mm.  long,  170  to  250  /^  wide, 


IQ4 


HISTOLOGY. 


containing  many  nuclei  and  staining  more  deeply  than  the  surrounding 
tendon.     They  are  enclosed  in  sheaths  of  ordinary  connective  tissue. 


Medullated     Muscle 
nenes.        fibers. 


"eniiiiial  ramification.     Tendon  bundle 


_-  _  Medullated  nerve  fiber. 


Muscle  fibers. 


>y^l 


Fig.  122.  —  Muscle 
Spindle  of  an 
Adult  Cat.  X  135- 


Fig.  123. — Tendon  Spindle  of  an  Adult  Cat.    X  So. 


"^     A.xis  cylinder. 


Fig.  124.— The  Left  Portion  of  Fig.  123.  X  345- 

The  few  nerve  fibers  which  terminate  in  a  tendon  spin- 
dle lose  their  sheaths  and  branch  freely,  ending  in  club- 
shaped  enlargements  (Figs.  123  and  124).  They  are 
found  in  all  tendons  and  serve  to  transmit  the  sensation 
of  tension,  being  active  in  connection  with  coordinated 
movements. 

In  connective  tissue,  sensory  nerves  may  either  end 
free  or  surrounded  with  a  connective  tissue  capsule.    In 
the  subcutaneous  tissue  near  the  coils  of   the   sweat 
glands,  and  in  the  corium  of  the  fingers  and  toes,  there 
are  terminal  cylinders  [of  Ruffini]  which  resemble  tendon  spindles  in  the 


ENCAPSULATED    SENSORY   ENDINGS. 


105 


way  that  the  nerves  ramify  (Fig.  125).     These  cyhnders  lack  the  distinct 

capsules  which  characterize  the  nerve  corpuscles. 

Terminal  corpuscles  are  nerve  endings  consisting  of  a  coarse  nerve 

fiber  or  knot  of  small  branches  surrounded  by  a  semifluid  intercellular 

substance  (which  is  granular  in  preserved  tissue)  and  enclosed  in  a  con- 
nective tissue  capsule.  The  terminal 
ramifications  of  the  nerve  show  irregular 
swelhngs  or  varicosities,  such  as  are  found 
along  terminal  nerve  fibers  generally  and 
which  are  not  considered  artificial.  Some 
authorities  describe  the  interlacing  termi- 
nal branches  as  ending  bhndly,  but  others 
beheve  that  they  unite  so  as  to  make  a 
network.  Often  more  than  one  fiber  en- 
ters a  corpuscle  and  it  has  been  suggested 
that  they  include  afferent  and  efferent 
fibers.  Generally  the  connective  tissue 
sheaths  of  the  entering  fibers  blend  with 
the  capsule  of  the  corpuscle,  and  the  mye- 
lin sheaths  are  lost  just  inside  the  capsule. 
Terminal  corpuscles  have  been  grouped 
as  tactile,  genital,  bulbous,  articular,  (cylin- 
drical), and  lamellar. 


Fig.  125. — Terminal    Cylinder. 

(After   Ruffini,    from    Ferguson's 

Histology. 
gH,    Medullary    sheath;    il,    terminal 

ramifications  of  the  axis  cylinder; 

L,  connective  tissue. 


Fig.  1C.6. — Tactile  Corpuscle  from  a  Perpendicular 
Section  of  the  Great  Toe  of  a  Man  Twenty- 
five  YE.A.RS  Old.    X  560. 

tirMedullated  nerve  fibers;  e,  varicosities ;  h,  connective 
tissue  sheath.     The  nuclei  are  invisible. 


Tactile  corpuscles  [of  Meissner]  are  elhptical  structures,  40-100  fJ- 
long  and  30-60  ij-  broad  (Fig.  126).  They  are  characterized  by  transverse 
markings  due  to  the  corresponding  elongation  of  their  capsule  cells  and 
nuclei.  From  one  to  five  medullated  fibers  enter  the  lower  end  of  a  tactile 
corpuscle,  losing  their  sheaths  on  entering.     Some  fibers  may  pass  straight 


io6 


HISTOLOGY. 


through  the  axis  of  the  corpuscle,  the  others  making  spiral  turns  about 
them  before  breaking  up  into  numerous  varicose  branches.  Tactile 
corpuscles  are  found  in  certain  of  the  connective  tissue  elevations  {papillae) 
just  beneath  the  epidermis,  being  especially  numerous  in  the  soles  and 

palms  (23  in  i  sq.  mm.)  and  at  the 
finger  tips;  also  "in  the  nipple,  bor- 
der of  the  eyehds,  lips,  glans  penis 
and  clitoris." 

Genital    corpuscles    are    large, 
round  or  oval  bodies  60-400  /-<  long 


Fig.  127. — Genital  Corpuscle  fro.m  the 
Gi.ANS  Penis  ok  Man.  Methylene 
Blue  Stain.  (After  Dogiel,  from  Bohm 
and  von  Davidoff.) 


Fig.  12S.— Bulbous  Corpuscle  from  the  Con- 
junctiva OF  Man.  Methylene  Blue 
Stain.  (After  Dogiel,  from  Bohm  and  von 
Davidoff.  i 


(Fig.  127)  which  may  receive  as  many  as  ten  nerve  fibers.  These  ramify 
and  send  branches  to  neighboring  corpuscles  and  also  to  the  epidermis. 
The  genital  corpuscles  are  deeply  placed  beneath  the  epithehum  of  the 
glans  penis  and  clitoris  and  the  adjoining  structures. 

Bulbous  corpuscles  [of  Krause]  are  smaller  than  the  genital  corpuscles 
and  are  most  numerous  (1-4  in  a  sq.  mm.)  in  the  superficial  connective 

tissue  of  the  glans  penis  and  clitoris. 
Similar  structures,  either  round  or 
oval,  arc  found  in  the  conjunctiva 
and  "edge  of  the  cornea,  in  the  lips 
and  hning  of  the  oral  cavity,  and 
probably  in  other  parts  of  the  co- 
rium."  They  vary  in  length  from  20 
to  100  I'-;  they  have  thinner  capsules 
and  receive  fewer  nerves  than  tlic 
genital  corpuscles  which  they  resemble  (Fig.  128).  The  articular  cor- 
puscles, found  near  the  joints,  belong  in  the  same  category. 

Cylindrical  corpuscles  [cylindrical  end  bulbs  of  Krause]  contain  a 
single  axial  nerve  fiber  with  few  or  no  branches,  terminating  in  a  knob-hke 
or  rounded  extremity  (Fig.  129).     The  fiber  is  surrounded  by  a  scmifiuid 


''^'^  *  -    -^ 


Fig.  129.— Cylindrical  Corpuscles,  fro.m  1n- 
ter.muscular  Septum  of  Cat.  Methylene 
Blue  Stai.m.    (Huber.) 


MOTOR   EXDIXGS. 


107 


Axis  cvlinder. 


Inner  bulb. 


substance,  sometimes  described  as  an  inner  bulb,  and  this  is  enclosed  in 
a  few  concentric  layers  of  cells  which  are  continuous  with  the  sheath  of 
the  nerve.  Cyhndrical  corpuscles  are  found  in  the  mucous  membrane 
of  the  mouth  and  in  the  connective  tissue  of  muscles  and  tendons. 

Lamellar  corpuscles  [Pacinian  corpuscles]  are  macroscopic  elhptical 
structures,  2-4.5  nini-  long  and  1-2  nrni.  wide  (Fig.  130).  They  may 
have  as  many  as  fifty  concentric  layers  of  flattened  capsule  cells  between 
which  there  are  spaces  containing  fluid.  A  single  large  nerve  fiber  enters 
one  end  of  the  corpuscle  and  loses  its- myelin  as  it  traverses  the  lamellae. 
It  extends  through  the  semifluid  inner  bulb  without  obvious  branches, 
sometimes  being  flattened  and  band-like;  it  may  fork  at  its  further  end  or 
form  a  coil  of  branches.  Special  methods  have  shown  that  the  axial 
fiber  may  possess  many  short  lateral 
branches  ending  in  knobs,  and  that 
one  or  more  dehcate  fibers  may  enter 
(or  leave j  the  corpuscles  in  addition  to 
the  large  one  just  described;  they  form 
a  net  surrounding  the  axial  fiber.  A 
small  arter}'  may  pass  into  the  corpus- 
cle beside  the  nerve  and  supply  the 
lamellae  with  capillaries.  Lamellar 
corpuscles  are  abundant  in  the  subcu- 
taneous tissue  of  the  hand  and  foot  and 
occur  in  other  parts  of  the  skin,  in  the 
nipple,  and  in  the  territory  of  the  pu- 
dendal nen-e ;  they  are  found  near  the 
joints  (particularly  on  the  flexor  sidej 
and  in  the  periosteum  and  perimysium, 
the  connective  tissue  around  large  blood 

vessels  and  nerv^es,  and  in  the  tendon  sheaths ;  also  in  the  serous  menj- 
branes,  particularly  in  the  mesenteries.  As  they  are  usually  cut  obhquely 
or  transversely  the  student  should  expect  to  find  the  lamellae  completely 
encirchng  the  inner  bulb. 

Motor  Exdixgs.  The  motor  nerv^e  endings  are  the  terminations  of 
efferent  nen-es  in  contact  with  smooth,  cardiac  or  striated  muscle  fibers. 
The  nerves  to  the  smooth  muscles  are  a  part  of  the  s}Tiipathetic  system. 
They  are  non-medullated  fibers  which  branch  repeatedly,  forming  plexuses. 
From  the  plexuses  very  slender  varicose  fibers  proceed  to  the  muscle 
cells,  in  contact  with  the  surface  of  which  they  end  in  one  or  two  terminal 
or  lateral  nodular  thickenings.  Probably  each  muscle  cell  receives  a  nerve 
termination.     Except  that  the  nerve  endings  in  heart  muscle  are  a  httle 


Fat  cells. 


Fig.  13c. — Small  La.mellar  Corpusclk  from 
THE  Mesentery  of  .\  Cat.    X  50. 

The  cells  lining  the  capsules  can  be  recognized 
by  their  shaded  nuclei.  The  myelin  of  the 
nerve  fiber  mav  be  traced  to  the  inner  bulb. 


io8 


HISTOLOGY. 


larger,  often  provided  with  a  small  cluster  of  terminal  nodules,  they  are 
like  those  of  smooth  muscle.  They  belong  with  the  sympathetic  system. 
The  accessory  fibers  of  the  vagus  which  enter  the  cardiac  plexuses,  are  not 
known  to  terminate  upon  the  muscle  fibers. 


Sensory- 
nerve  fibers 


Muscle  fibers 


,t.r. 


Ner\e  fiber 
bundle. 


Fig.  131. — Motor  Nerve  Endings  of  Intercostal  Mlscle  Fibers  of  a  Rabbit.     X  150. 

Striated  muscles  are  innervated  by  the  neuraxons  of  the  ventral  roots, 
which  grow  out  from  cell  bodies  remaining  within  the  central  system. 
These  neuraxons,  as  medullated  fibers,  extend  through  the  spinal  and 
certain  cerebral  nerves  to  the  muscles.  They  form  plexuses  of  medullated 
fibers  in  the  perimysium,  from  which  branching  medullated  fibers  pass  on  to 
the  muscle  (Fig.  131).     Each  muscle  fiber  receives  one  of  these  branches,  or 

sometimes  two  placed  near  together. 
They  are  usually  implanted  near  the 
middle  of  the  muscle  fiber.  The 
connective  tissue  sheath  of  the  nerve 
blends  with  the  perimysium;  the 
neurolemma  is  said  to  be  continu- 
ous with  the  sarcolemma,  the  nerve 
having  become  attached  to  the  em- 
bryonic muscle  fiber  before  the  sarco- 
lemma had  developed.  Under  this 
membrane  the  myelin  sheath  ends  abruptly,  and  the  fiber  ramifies  in  a 
granular  mass  considered  to  be  modified  sarcoplasm.  It  may  contain 
muscle  nuclei.  This  granular  mass  with  the  nerve  ending  appears  as  a 
distinct  elevated  area,  estimated  to  average  from  40  to  60  n  in  diameter, 
and  has  been  named  the  motor  plate.  A  surface  view  and  a  section  of  a 
motor  plate  are  shown  in  Fig.  132. 


y^^^m^^^^ 


A 


B 


Fig.  132.— Motor  Plates. 
A,  Surface  view,  from  a  guinea  pig;  B,  vertical 
section,  from  a  iR-ilgehog.  (After  Bohm  and 
von  Uavidoff.)  g.,  Granular  substance  of  the 
motor  plate;  m.,  striated  mnscle ;  n.,  nerve 
fiber  ;  t.  r.,  terminal  ramifications  of  the  nerve 
fiber. 


GANGLIA. 


109 


Ganglia.  The  ganglia  are  enlargements,  usually  macroscopic,  oc- 
curring in  the  course  of  the  peripheral  or  sympathetic  nerves.  They  always 
consist  of  nerv'e  fibers  between  which  there  are  rows  or  rounded  groups 
of  the  bodies  of  nerve  cells.  Nerve  cell  bodies  vary  in  diameter  from 
4  to  150  iJ..  Thus  they  include  some  of  the  largest  cells  in  the  body.  Each 
has  a  single  round  or  oval  nucleus  which  appears  vesicular  because  of  its 
small  amount  of  chromatin.  It  contains  usually  one  large  round  nucleolus. 
These  nuclei  are  so  characteristic  that  the  student  should  soon  learn  to 
recognize  them.  Near  the  nucleus  the  centrosome  has  been  detected, 
sometimes  represented  by  a  number  of  granules;  but  mature  nerve  cells 
never  divide  and  if  destroyed   they  cannot  be   replaced.     In  ordinary 


Blood  vessel. 


i'at.  Ganglion  cells 


Nerve  fibers. 


Perineurium. 


Fig.  133.— Longitudinal  Section  through  a  Spinal  Ganglion  of  a  Cat.    X  18. 


specimens  the  protoplasm  is  densely  granular.  There  is  no  ceU  membrane. 
Except  in  the  embryo,  nerve  cells  all  have  one  or  more  processes;  and 
according  to  the  number  of  these,  one,  two,  or  several,  they  are  designated 
unipolar,  bipolar,  and  multipolar  respectively.  The  processes  cannot  be 
traced  in  ordinary  specimens  because  of  their  thick  entanglement  with 
those  of  other  ceUs.  In  studying  them  the  special  methylene  blue  and 
silver  (Golgij  methods  are  employed.  If  pieces  of  very  fresh  nerve  tissue 
are  placed  in  a  dilute  solution  of  methylene  blue,  after  an  hour  or  more 
the  processes  of  certain  cells  are  stained  so  that  they  can  be  followed  satis- 
factorily.    By  Golgi's  silver  method  a  black  precipitate  occurs  in  and  on 


no  HISTOLOGY. 

individual  nerve  cells,  following  their  branches  to  their  smallest  subdivisions, 
whereas  similar  adjoining  cells  are  entirely  unaffected.  This  extraordinar)' 
method  is  of  the  greatest  value,  but  it  is  capricious  and  the  silhouettes 
produced  are  in  part  coarse  and  artificial  appearances. 

The  ganglia  are  surrounded  by  connective  tissue  sheaths,  continuous 
withj^^the^erineurium,  which  send  prolongations  into  their  interior  to 
invest  the  cell  bodies  and  libers.  They  contain  an  abundance  of  blood 
vessels  so  that  a  cell  body  may  be  surrounded  with  capillaries.  The  spinal 
and  sympathetic  gangha  will  be  described  in  turn. 

Spinal  ganglia  are  found  on  the  dorsal  roots  of  spinal  nerves;  similar 

Cross  section  of  medullated  nerve  fibers. 

I  /^^*®^ NiKkUcd  shLith. 

^ /T  y         ^ Iiotoplisn  J 


Lona;itu(iiinl  \  icw  ^  ^  ^^         ^^& 

of  medullated 

ner\^  f.b<_r.,.  buita-i.  \  icw  of 

nucleated  sheatli- 
FiG.  134. — From  a  Cross  Srction  ok  the  Semilunar  Ganglion  of  Man.     X  240. 
The  cell  processes  cannot  be  seen.     At  X  the  protoplasm  of  the  ganglion  cell  has  retracted  and  simulates 
a  process.     In  the  axis  01  the  transversely  cut  nerve  fibers  the  axis  cylindei  s  are  seen  in  section. 

structures  occur  on  the  dorsal  roots  of  the  cerebral  nerves.  The  general 
relations  of  the  cell  bodies  and  fibers  are  shown  in  Fig.  133,  a  longitudinal 
section  through  the  dorsal  and  ventral  roots.  Fig.  134,  from  the  semilunar 
[Gasserian]  ganglion  of  the  trigeminus,  shows  the  component  structures 
on  a  large  scale.  In  the  upper  part  of  the  figure  there  are  characteristic 
nerve  cells  such  as  have  the  T-shaped  process,  the  development  of  which 
from  bipolar  cells  has  already  been  described.  The  processes  are  not 
seen  in  the  section.  Each  of  these  cell  bodies  is  surrounded  by  a  nucleated 
capsule  said  to  be  continuous  with  the  neurolemma  of  its  fiber.  Fig.  135 
shows  one  of  these  cell  bodies  containing  canaliculi  which  have  been 
regarded  as  nutritive  passages  from   the  exterior,  and  as   secretory  or 


SPINAL    GANGLIA. 


Ill 


Nucleus 


excretory  vacuoles.  Fig.  136  is  a  similar  cell  containing  a  reticular  network 
within  its  protoplasm.  Nerve  fibers  branch  over  the  outer  surface  of 
ganghon  cells,  forming  pericapsular  and  pericellular  nets  or  baskets, 
and  have  been  said  to  penetrate  the  protoplasm.  This,  however,  is  denied, 
and  such  formations  as  are  represented  by  Fig.  136  are  thought  not  to  pass 
outside  of  the  cell.  Ganglion  cells  often  contain  areas  of  yellow  or  brown 
fatty  pigment  granules  which  increase  with  age. 

The  results  of  special  investigations  of  the  course  of  the  dorsal  ganglion 
fibers,  made  by  the  methylene  blue  method,  are  sho^^^l  in  the  diagram, 
Fig.  137.  The  large  round  cells  (i)  give  rise  to  a  single  spirally  twisted 
process  which  begins  at  the  apex  of  a  conical  elevation  on  the  cell  body. 
The  spiral  fiber  has  a  neurolemma  and  acquires  a  myelin  sheath.  It  may 
give  off  collateral  branches  (2).  At  the  first  or  second  node,  sometimes 
further  on,  it  divides  into 
a  cellulipetal  or  afferent 
branch,  which  is  an  axis 
cylinder  with  a  per- 
ipheral sensory  ending, 
and  a  cellulifugal  or 
efferent  branch  which 
enters  the  spinal  cord 
(Fig.  III).  The  cellulip- 
etal fiber  may  have  a 
branch  in  the  dorsal 
ramus  and  another  in  the 
ventral  ramus  (2);  and 
the  celluKfugal  fiber  may 

fork  near  the  cell  body  (3)  or  at  some  distance  from  it  (2).  Besides  the 
large  cells  there  are  similar  smaller  ones,  the  fibers  from  which  have  Httle 
or  no  medullary  sheaths  (4).  It  is  to  be  noted  that  in  all  these  forms  the 
cell  bodies  become  virtually  appended  to  single  fibers,  which  in  relation  to 
the  central  nervous  system  are  afferent. 

A  second  type  of  cell,  which  occurs  less  frequently,  is  the  round 
unipolar  form  (6)  the  process  of  which  divides  into  many  medullated 
branches.  After  losing  their  myelin  these  form  pericapsular  and  peri- 
cellular ramifications  around  the  cell  bodies  of  the  first  type.  Each  of 
the  latter  is  in  relation  with  branches  from  several  cells  of  the  second  type. 
A  third  form  is  a  multipolar  cell  with  two  medullated  fibers  which  are 
thought  not  to  pass  beyond  the  hmits  of  the  ganghon  (7). 

Fibers  from  sympathetic  cells  enter  the  ganglion  from  the  periphery 
and  branch  about  the  blood  vessels  and  cells  of  the  second  t}'pe.     Through 


■■^. 


Sheath. 


Fig.  135.  —  Spinal  Ganglion 
Chll  of  an  Adult  Cat. 
X  430- 


Reticular  apparatus. 


Fig.  136.  —  Spinal  Ganglion 
Cell  of  .a.  Xewborx  Kit- 
ten.) (Copied  after  Golgi.) 


112 


HISTOLOGY, 


the]cells  of  the  second  type  a  few  sympathetic  fibers  are  put  in  communi- 
cation with  a  large  number  of  T-cells.  Apparently  in  mammals  there  are 
no  fibers  which  traverse  the  spinal  ganglion  without  entering  into  relation 
with  its  cell  bodies.     The  observation  that  there  are  types  of  spinal  gangUon 


Spinal  ganglion. 


Motor  cell  of 
anterior  horn  (i 
the  spinal  cord 


Blood  vessel. 


Dorsal  ramus. 


Visceral  ramus. 
Cell  of  a  sympathetic  ganglion. 


Fig.  137. — DiAOR.A.M  ok  the  Nervoi's  Eleme.nts  of  a  Spinal  Gancjlkjn,  Based  upon 
Methylene  Blue  Preparations. 
The  sensory  fibers  are  represented  by  continuous  lines,  the  sympathetic  fibers  by  dotted  lines,  the  motor 
fibers  by  linear  series  of  dashes.     The  medullary  sheaths  of  the  motor  fibers  of  the  \entral   root  have 
not  been  drawn. 


cells  with  processes  confined  within  the  ganghon,  and  that  some  of  the  cells 
have  non-medullated  fibers,  accords  with  the  fact  ascertained  by  counting, 
that  the  ganghon  may  contain  about  six  times  as  many  cells  as  there 
are  medullated  fibers  in  the  dorsal  root. 

Sympathetic  ganglia  consist  of  smaller  cell  bodies,  often  pigmented, 


SYMPATHETIC    GAXGLIA. 


113 


and  sometimes  having  two  nuclei,  and  of  fibers  some  of  which  merely 
traverse  the  gangha.  The  cells  are  enveloped  in  nucleated  sheaths.  They 
include  three  types  of  multipolar  cells  shown  in  Fig.  138.  Most  of  the 
cells  are  of  rounded  oval  form,  often  flattened,  having  stellate  spiny 
dendrites   and   a  non-medullated   neuraxon  with  very  slender   collateral 


Motor,        Sensory  spinal 
"\     ,  ^  nerve  fiber. 


Pericellular  plexus 


Capsule 


View  in  section 

of  pericapsular 

plexus. 

Surface  view 

of  pericapsular 

plexus. 


Neuraxons. 


Sympathetic 
nerve. 


Smooth 
muscle  fibers 


Lamellar  corpuscle. 


Fig.  138.— Diagram  of  the  Elements  of  Two  Sympathetic  G.4.nclia,  Based  upon 
Methylene  Blue  Prep.a.rations. 


branches  (i).  These  are  motor  cells,  and  their  neuraxons  terminate  in 
contact  with  smooth  muscle  cells.  The  second  type  (2),  possibly  sensory, 
includes  rounded  polygonal  cells  with  slender  dendrites  which  extend  in 
the  sympathetic  nerves  even  to  the  neighboring  gangha.  Their  neuraxons 
may  acquire  myehn  sheaths  at  some  distance  from  the  cell  body  or  may 

remain  non-medullated.     They  pass  to  other  gangha  but  their  termination 

8 


114  HISTOLOGY. 

is  unknown.  Cells  of  the  third  type  (3)  are  few  in  the  large  ganglia  and 
are  not  found  in  small  ones.  They  have  long  dendrites  which  form  a  "general 
peripheral  plexus  "  but  do  not  extend  beyond  the  limits  of  the  ganghon. 
Their  neuraxons  enter  the  sympathetic  ner\^es  as  non-medullated  fibers, 
the  destination  of  which  is  unknown.  Sympathetic  gangha  contain  also 
stellate  connective  tissue  cells,  and  chromaffine  cells  to  be  considered 
presently.  The  ganglia  may  be  traversed  by  sensory  medu Hated  fibers 
to  lamellar  corpuscles,  and  by  medullated  motor  fibers  which  lose  their 
myehn  sheaths  and  have  non-medullated  collateral  branches.  The  motor 
fibers  and  their  collaterals  terminate  in  rather  coarse  pericellular  ramifi- 
cations about  the  sympathetic  cells  of  the  motor  type.  There  are  other 
nerve  fibers,  non-medullated  and  varicose,  which  form  pericapsular  plexuses, 
and  these  are  considered  to  be  branches  of  sympathetic  cells. 

Paraganglia  are  masses  or  cords  of  cells  which  originate  in  the  em- 
bryonic sympathetic  gangha,  and  are  characterized  by  being  colored 
yellowish  brown  by  preserving  fluids  containing  chromic  acid  or  chromium 
salts.  The  cells  are  therefore  called  chromaffine  (meaning  that  they  have 
an  affinity  for  chromium,  and  not,  hke  'chromatic  material,'  for  coloring 
matters  generally).  The  paragangha  are  either  closely  or  shghtly  con- 
nected with  the  sympathetic  nerves.  In  the  latter  case  they  are  applied 
to  large  vessels,  and  in  the  fetus,  between  the  branches  of  the  spermatic 
vessels,  to  the  paroophoron  and  paradidymis.  The  glomus  caroticum  at 
the  bifurcation  of  the  carotid  artery,  and  the  glomus  coccygeum  associated 
with  the  median  sacral  artery,  are  knots  of  vessels  both  of  which  contain 
clumps  of  chromaffine  cells.  The  organs  discovered  by  Zuckerkandl  at 
the  origin  of  the  inferior  mesenteric  artery  may  be  classed  with  them. 
Single  chromafiine  cells,  or  small  groups  of  them,  occur  dift\isely  in  the 
sympathetic  gangha  and  nerves.  The  entire  medulla  of  the  suprarenal 
gland  in  the  higher  vertebrates  is  composed  of  them.  Since  the  extract 
of  such  cells,  on  intravenous  injection,  causes  a  marked  increase  in  the 
blood  pressure,  the  chromaffine  cells  are  considered  to  secrete  into  the 
blood  a  specific  substance  which  maintains  the  normal  tonus  of  the  vessel 
•  walls. 

Spinal  Cord  (Medulla  spinalis).  Development.  The  early  develop- 
ment of  the  medullary  tube  has  been  sho^\Tl  in  Fig.  109,  p.  92.  The 
tube  at  first  consists  of  separate  cells  but  these  soon  unite  to  form  a  syncy- 
tium. Those  nuclei  of  the  syncytium  which  border  upon  the  central 
canal  divide  repeatedly  by  mitosis  and  many  of  them  are  forced  outward 
radially.  The  protoplasm  of  the  syncytium  increases  more  rapidly  than 
the  nuclei,  and  forms  a  non-nucleated  network  at  the  periphery  of  the 
tube;   this  is  the  white  layer  [sometimes  called  mantle  layer].     The  fibers 


SPIXAL    COEJ). 


Ii: 


i 


-ep. 
-wJ. 


from  the  spinal  ganglia  enter  its  dorsal  portion  and  grow  up  and  doT^Ti  the 
medullary  tube  through  its  meshes,  thus  forming  the  oval  bundles.  Mean- 
while the  nucleated  layer  becomes  di\dsible  into  two  portions,  a  thick 
ependymal  layer  composed  of  undifferentiated  cells  around  the  central 
canal;  and  a  gray  layer  [mantle  layer]  composed  of  cells  which  have 
moved  outward  and  become  partly  differentiated.  The  gray  layer  is  at 
first  triangular,  being  thick  ventrally  and  narrow  dorsally.  It  consists  of 
two  sorts  of  cells,  the  neuroglia  cells  (gha  cells),  which  are  the  cells  of  the 
protoplasmic  syncytium;  and  the  nerve  cells  (in  their  young  stage,  called 
neuroblasts),  which  are  imbedded  in  the  neurogha  network  and  send  out 
processes  to  ramify  among  its  meshes.  The  neuraxons  of  the  motor 
cells  grow  out  from  neuroblasts  in  the  ventro-lateral  part  of  the  gray  layer; 
after  crossing  the  white  layer,  they 

pass  out  of  the  medullary  tube  as  'i™- 

fibers    of  the  ventral  roots.     This 

stage  of  development  is  shown  in  /:  .    ^^ 

Fig.  109,  E.  Blood  vessels  are  seen 
growing  into  the  tube  under  the 
dorsal  roots  and  near  the  ventro- 
median  line.  They  carry  some 
connective  tissue  cells  with  them, 
to  mingle  with  the  neuroblasts  and 
neurogha,  both  of  which  are  ecto- 
dermal. 

Fig.  139  represents  a  later  stage 
in  which  the  form  of  the  adult  cord 
is  clearly  suggested.  The  walls  of 
the  dorsal  portion  of  the  central 
canal  have  fused  and  disappeared 
so  that  the  canal  is  reduced  in  size. 

It  is  surrounded  by  an  epend^Tnal  layer  which  is  becoming  thinner,  since  its 
cells  are  being  added  to  the  gray  layer  faster  than  they  are  replaced  by 
mitosis  of  the  inner  cells.  The  gray  layer  in  the  preceding  stage  showed 
two  ventral  protuberances,  one  on  each  side.  These  extend  the  length 
of  the  cord  and  are  knoT^m  as  the  ventral  columns  [horns].  In  the  present 
stage  in  addition  to  these,  there  are  two  dorsal  columns  [horns]  which  have 
been  formed  by  the  dorsal  prohferation  of  the  ependymal  layer.  As  a 
whole  the  gray  is  shaped  hke  an  H.  That  portion  which  extends  from 
side  to  side  beneath  the  central  canal  is  the  ventral  gray  commissure.  The 
white  layer  has  become  wider.  Its  neurogha  network  has  a  predominant 
radial  arrancrement.     Nuclei  are  found  in  its  strands  of  neuroglia  which 


v.m.f. 


'.cNv.< 


Fig.  139. — Spinal  Cord  of  a  Rabbit  Embryo 
OF  20  Days. 
C.  C,  Central  canal ;  d.  c,  dorsal  column  ;  d.  m.  S., 
dorsal  median  sulcus;  d.  P.,  dorsal  root;  ep., 
ependymal  layer;  v.  c,  ventral  column  ;  v.  g.  c, 
ventral  gray  commissure;  v.  m.  f.,  ventral 
median  fissure ;  v.  r.,  ventral  root ;  v.  w.  c,  ven- 
tral white  commissure;  w.  I.,  white  layer  (lat- 
eral funiculus). 


ii6 


HISTOLOGY. 


have  become  fibrous,  but  it  lodges  no  nerve  cell  bodies.  It  is  permeated 
with  the  processes  of  nerve  cells,  the  bodies  of  which  remain  within  the  gray 
layer,  or  the  spinal  gangha.  On  the  outer  surface  of  the  cord  there  are 
longitudinal  grooves  which  form  the  boundaries  of  certain  subdivisions 
of  the  white  layer.  These  grooves  are  the  dorso-median  sulcus;  the 
dorso-lateral  sulcus,  along  which  the  dorsal  roots  enter  the  cord;  the  ventro- 
lateral sulcus,  along  which  the  ventral  roots  leave  the  cord ;  and  the  ventro- 

Dorsal  Median ~j  Portion  of 

median      Dorsal        I  y     dorsal 

Entrance  zone.      seiitum.    funiculus.    |  Lateral)        root.  Dorsal  root. 


Zona 
terminalis. 


Zona 
spongio.sa. 


Substantia 
gelatinosa. 


Dorsal  column. 


Formatio     / 
reticularis.  /', 


E.xternal 

limiting 

membrane. 


Lateral 
funiculus. 


Dorso-  /        v'm™ 
lateral. '  ^^ 


•t.-r- 


iS*:^ 


Ventro-medial.      ^vti^^i 
Groups  of  nerve  cells.  / 

Ventral  root. 


^"*g*«S3- 


Wliite  Ventral  Ventral  funiculus, 

commissure.      median 
fissure. 


N'entral  column. 


Central  canal. 


Fig.  140. — Cross  Section  of  the  Lumrar  Enlargement  of  the  Human  Spinal  Cord.    X  8. 


median  fissure,  which  unlike  the  others  becomes  a  very  deep  narrow 
depression.  Between  these  four'  grooves  the  white  substance  on  either 
side  of  the  cord  forms  the  dorsal,  the  lateral,  and  the  ventral  funiculi. 
Each  dorsal  funiculus  receives  the  entering  fibers  from  the  dorsal  roots 
on  one  side  of  the  cord;  it  represents  the  oval  bundle  which  has  enlarged 
and  been  folded  in  toward  the  median  dorsal  Hne.  Later  a  dorsal  median 
septum  becomes  more  evident  separating  the  two  dorsal  funicuK.     Ven- 


SPINAL   CORD. 


117 


trally  there  is  a  narrow  layer  of  white  substance  extending  from  one  side 
of  the  cord  to  the  other;  this  is  the  ventral  white  commissure. 

In  the  adult  cord  (Fig.  140)  the  central  canal  is  usually  reduced  to 
a  cavity  0.5  to  i.o  mm.  broad;  sometimes  it  is  obliterated.  The  canal  is 
surrounded  by  the  ependyma  which  appears  as  a  single  layer  of  neurogha 
cells.  Around  the  ependyma  is  the  central  gray  substance,  containing 
special  neurogha  cells  to  be  described  later.  In  addition  to  the  ventral 
gray  commissure  of  the  younger  stage,  there  is  now  a  dorsal  commissure, 
by  which  the  vertical  portions  of  the  gray  H  are  united  dorsal  to  the  central 


Fig.  141.— Neuroglia  Cells  and  Fibers  from  the  Spinal  Cord  of  ax  Elephant. 

{Hardesty ,—irovn.  Ferguson's  Histology.) 

The  letters  indicate  the  neuroglia  cells.     I,,  a  leucocyte.     Benda's  stain.     X  940. 


canal.  Besides  the  dorsal  and  ventral  columns,  a  lateral  column  may  now 
be  recognized  as  a  bulging  of  the  ventral  colunm  on  a  hne  with  the  central 
canal.  Lateral  columns  are  most  e^ddent  in  the  upper  thoracic  part  of  the 
cord.  On  the  lateral  side  of  the  dorsal  column  there  is  a  network  of 
strands  of  gray  substance  called  the  reticular  jormation  (formatio  retic- 
ularis). Near  the  dorsal  coramissure  in  the  dorsal  column  there  is  an 
important  group  of  nerve  cell  bodies  named  the  dorsal  nucleus  [column  of 
Clark].  ('Nucleus'  is  a  term  apphed  to  many  such  groups  of  ceU  bodies 
in  the  brain.)  The  dorsal  nucleus  extends  through  the  thoracic  cord 
and  is  well  defined  in  the  anterior  lumbar  portion;  it  is  not  wholly  absent 


HISTOLOGY. 


Blood  vessel 


from  other  parts  of  the  cord.  Toward  the  tip  of  the  dorsal  cokimn  there 
is  a  macroscopic,  apparently  gelatinous  mass  called  the  gelatinous  substance 
(substantia  gelatinosa);  and  dorsal  to  this  there  occur  successively  the 
spongy  zone,  and  the  terminal  zone  (zona  spongiosa  and  zona  terminalis). 
The  latter  consists  chiefly  of  nerve  fibers  running  lengthwise  of  the  cord. 
The  dorsal  median  septum,  generally  described  as  formed  of  compressed 
strands  of  neuroglia,  is  well  marked;  it  resembles  the  ventral  median 
fissure  since  the  walls  of  the  latter  have  been  brought  so  close  together. 

Structure  of  the  cord.  From  the  preceding  account  of  the  develop- 
ment and  topography  of  the  cord,  it  is  evident  that  there  are  three  layers 
to  be  examined,  the  white  layer,  the  gray  layer,  and  the  ependyma;  these 

may  be  considered  in 
turn. 

The  white  sub- 
stance [matter]  con- 
sists of  a  syncytial 
framework  of  neurog- 
lia through  which 
pass  blood  vessels? 
and  nerve  fibers 
mostly  meduUated. 
The  myehn  sheaths 
of  the  latter  produce 
the  very  white  macro- 
scopic appearance  of 
this  layer  when  freshly 
cut.  The  nature  of 
the  neuroglia  syn- 
cytium is  seen  in  the  longitudinal  section,  Fig.  141.  Stiff  fibrils 
have  developed  in  its  exoplasm,  and  they  are  continuous  from  one 
cell  territory  to  another.  As  the  nerve  fibers  which  occupy  the  neuroglia 
meshes  increase  in  number,  and  in  size  by  becoming  medullated,  the 
neurogha  nuclei  surrounded  by  protoplasm  are  compressed  into  stellate 
forms  (Fig.  144,  A).  In  the  Golgi  preparations  they  appear  as  in  Fig.  142, 
and  are  described  as  long  rayed,  and  short  rayed  or  mossy  cells.  These 
forms  represent  clumps  of  neuroglia  fibers,  sometimes  clogged  with 
precipitate,  in  the  center  of  which  there  may  or  may  not  be  a  nucleus. 
Fig.  143  shows  the  appearance  of  the  neuroglia  net  in  ordinary  sections. 
Over  the  outer  surface  of  the  cord  it  makes  a  dense  feltwork,  generally 
free  from  nerves.  It  has  been  called  the  external  limiting  membrane. 
Outside  of  it  is  a  very  vascular  connective  tissue  layer,  the  pia  mater.    The 


Short  raved  cells. 


Long-  raved  cells. 


Fig.  142.— Xecroglia  Cells  from  the  Brain  of  an  Adult  Man. 
Golgi  Method.    X  280. 


WHITE    AND    GRAY    SUBSTANCE. 


119 


figure  shows  a  prolongation  of  the  pia  mater,  containing  blood  vessels, 
into  the  white  substance.     It  has  not  been  estabhshed  beyond  doubt 
that  such  ingrowths  of  connective 
tissue  may  not  take  part  in  form- 
ing supporting  tissue  around  the 
nerves. 

The  nerve  fibers  of  the  white 
substance  vary  in  diameter,  the 
coarsest  being  found  in  the  ven- 
tral and  the  lateral  parts  of  the 
dorsal  funiculi;  the  finest  are  in 
the  median  parts  of  the  dorsal 
and  lateral  funiculi.  Elsewhere 
coarse  and  fine  ones  are  inter- 
mingled. Their  general  direc- 
tion is  parallel  with  the  long  axis 
of  the  cord.  Like  other  nerve 
fibers  they  consist  of  neuroplasm 
and  fibrillae.  Most  of  them  are 
medullated  and  in  cross  section 
the  myelin  often  forms  concen- 
tric rings.  iVlthough  a  few 
observers  have  described  nodes  it  is  generally  considered  that  there  are  no 
nodes  in  the  central  nervous  system.     During  the  development  of  the 

myelin,  fibers  have  been  found  en- 


White 
substance. 

External  limiting 
membrane. 

j^-;  _    _  • 

Cross  sections  of 

medullated 

nerve  fibers 

consisting  of — 

',,     ^     ■-..^  '"]" 

—  Axis  cylinder 

and 
"-  Medullary  sheath 

--7  Neuroglia  cells. 

Connective  tissue. 


7~^     Blood  vessels. 


Fig.  143.  —  From  k  Cross  Section  of  the  Human 
Spinal  Cord  in  the  Region  of  the  Lateral 
Funiculus.    X  iSo. 


r.'^l 


<^' 


circled  by  sheath  cells,  Fig.  144,  B. 
In  longitudinal  view,  these  sheath 
cells  are  seen  in  depressions  of  the 
myelin,  where  they  greatly  resem- 
ble the  neurolemma  cells  of  per- 
ipheral nerves.  With  the  increase 
of  myelin  they  become  very  slender 
and  can  seldom  be  detected  in  the 
adult.  It  is  ordinarily  stated  that 
the  medullated  fibers  of  the  central 
nervous  system  are  without  a  neuro- 
lemma. 

The  gray  substance  [matter]  is 

composed  of  a  neurogha  framework 

containing  capillary  blood  vessels  and  some  larger  ones,  together  with  the 

cell   bodies   and   non-meduUated   processes   of  many  nerve   cells.     The 


c  a.c 


Fig.  144. 
Neuroglia  cells  and  nerve  fibers  from  a  cross 
section  of  the  spinal  cord  of  an  elephant.  B, 
Neuroglia  cells,  nerve  fibers  and  sheath  cells, 
from  the  spinal  cord  of  a  pig,  2  weeks  after 
birth.  C,  Isolated  fiber  from  the  cord  of  21  cm. 
pig  embryo,  stained  with  osniic  acid.  (After 
Hardesty.)  a.  C,  Axis  cylinder;  my.,  myelin; 
n.,  neuroglia  nuclei;  n.  f.,  neuroglia  fibrils; 
S.  C,  sheath  cell. 


I20  HISTOLOGY. 

processes  run  in  every  direction.  It  differs  from  the  white  substance, 
therefore,  in  the  absence  of  myehn,  the  presence  of  nerve  cell  bodies  and 
the  confused  courses  of  the  nerve  fibers. 

The  cell  bodies  belong  with  three  types  of  cells.  The  largest  are  the 
motor  cells,  67  to  135  ij-  in  diameter,  which  form  a  group  in  the  ventral 
column.  (In  the  cervical  and  lumbar  enlargements  of  the  cord  (Fig.  140) 
the  group  is  divided  into  dorso-lateral  and  ventro-medial  portions.)  Cell 
bodies  like  those  of  the  motor  cells  are  represented  in  Figs.  145  and  146. 
The  former  shows  the  fibrillar  stmcture  of  their  protoplasm,  and  the  latter 
the  groups  of  granules,  chromatic  bodies  (Nissl's  bodies)  which  may 
occur  between  the  fibrils.  These  are  rounded  or  angular  masses  which 
are  not  limited  to  motor  cells.     They  become  reduced  or  disappear  with 

Nissl's  bodies. 


W 


Fig.  145.— Nerve  Cell  of  the  Spinal  Cord     Fig.  146.— Nerve  Cell  of  the  Spinal  Cord 
OF  A  Child.  X  430.  of  a  Dog.  X  600. 

fatigue,  in  old  age,  and  in  certain  diseases  and  poisonings.  It  is  supposed 
that  they  are  nutritive  rather  than  nervous  elements.  After  preservation 
in  alcohol  they  may  be  stained  with  methylene  blue.  In  the  motor  cells 
the  fatty  pigment  may  be  abundant,  but  often  in  ordinary  specimens 
these  special  features  are  invisible  and  the  protoplasm  seems  densely 
granular.  The  processes  of  the  motor  cells  are  dendrites,  which  may 
extend  into  the  ventral  and  lateral  funiculi,  and  even  into  the  dorsal 
funicuh,  and  neuraxons  which  leave  the  cord  in  the  ventral  roots  and 
proceed  to  the  striated  muscles.  The  neuraxon  begins  as  a  slender  non- 
medullated  fiber  at  the  tip  of  a  clear  'implantation  cone'  and  acquires 
its  myehn  sheath  as  it  crosses  the  white  layer.  Ordinarily  it  has  no 
collaterals;  when  present  they  are  very  small. 

Cell  bodies  of  the  second  type  are  more  numerous  and  smaller  than 
the  motor  cells.  They  occur  singly  and  in  groups  throughout  the  gray 
substance.  Their  dendrites  are  long  but  with  comparatively  few  branches. 
Their  neuraxons  give  off  many  collaterals  in  the  gray  substance  and  enter 


FIBER  TRACTS  OF  THE  CORD. 


121 


the  lateral  and  ventral  funiculi,  rarely  the  dorsal.  Sometimes  they  cross  to 
the  opposite  side  of  the  cord  through  the  gray  commissure  before  entering 
the  white  substance  (Fig.  147).  In  the  white  they  fork,  sending  processes 
up  and  down  the  cord.  These  give  off  collaterals  which  re-enter  the  cord 
and  branch  about  the  motor  cells,  the  main  fiber  terminating  hke  its  col- 
laterals. These  cells  put  the  different  levels  of  the  cord  in  communication. 
The  neuraxons  from  the  dorsal  nucleus  (Fig.  147)  differ  from  these  in  that 


v.Tn.f. 


Fig.  147. — Diagram  of  the  Spinal  Cord. 
The  principal  fiber  bundles  are  outlined  on  the  left ;  the  predominant  courses  of  the  nerves  within  them 

are  indicated  on  the  right. 
Dorsal  funiculus : 

f.  g.,  fasciculus  gracilis  [column  of  Goll]. 

f.  C,         "  cuneatus  [column  of  Burdach] . 

Lateral  funiculus  : 

f.  C.  I.,  fasciculus  cerebrospinalis  lateralis  [crossed  pyramidal  tract] . 

f.  C,  "  cerebellospinalis. 

f.  V.  s.,         ''  ventrolateralis  superficialis  [Gowers' tract]. 

f.  I.p.,         "  lateralis  proprius  [ground  bundle]. 

Ventral  funiculus  : 

f.  V.  p.,  fasciculus  ventralis  proprius. 

f.  c.  v.,         "  cerebrospinalis  ventralis  [direct  pyramidal  tract] . 

Columns, — d.  c,  dorsal ;  I.  c,  lateral ;  v.  C,  ventral. 

d.  n.,  dorsal  nucleus. 
Sulci, — d.  m.  s.,  dorsomedian  ;  d.  I.  s.,  dorsolateral  ;  v.  I.  S.,  ventrolateral ;  v.  m  f.,  ventromedian  fissure. 


their  neuraxons  go  to  the  cerebellum  in  a  bundle  called  the  fasciculus 
cerebellospinalis.  The  spindle  shaped  cells  of  the  zona  spongiosa  are  also 
of  the  second  type. 

The  third  t5^e  is  characterized  by  having  all  of  its  processes,  the 
dendrites  and  neuraxon,  remain  within  the  gray  substance.  The  neuraxons 
are  much  branched,  and  may  cross  to  the  opposite  side  of  the  cord. 

There  are  therefore  three  types  of  nerve  cells  in  the  gray  substance, 
namely,  (i)   the  cells  with  processes  which  enter  the  peripheral  nerves; 


122  HISTOLOGY. 

(2)  cells  with  processes  limited  to  the  central  nervous  system  and  extending 
through  its  white  substance  from  one  part  to  another;  and  (3)  cells  with 
processes  limited  to  the  gray  substance. 

The  fibers  of  the  central  nervous  system  are  the  processes  of  these 
three  t^-pes  of  cells  together  with  those  which  enter  from  the  peripheral 
ganglia.  These  fibers  are  arranged  in  bundles  or  fasciculi  as  they  traverse 
the  white  substance.  The  boundaries  of  the  bundles  are  not  indicated  in 
ordinary  sections  and  are  never  sharply  outlined.  They  have  been  deter- 
mined in  various  ways,  such  as  cutting  certain  parts  of  the  cord  and 
observing  in  sections  the  path  of  the  fibers  which  degenerate  and  lose 
their  myelin  in  consequence.  These  results  are  confirmed  by  the  exami- 
nation of  embryos  in  which  certain  fiber  tracts  develop  their  myelin 
sheaths  earher  than  others.  It  has  been  found  that  each  dorsal  funiculus 
includes  two  large  fasciculi,  the  cuneate  and  gracile,  respectively.  The 
cuneate  fasciculus  which  is  the  more  lateral,  receives  the  fibers  of  the 
dorsal  root.  In  it  they  divide  into  ascending  and  descending  fibers  and  give 
off  the  reflex  collaterals  to  the  motor  cells  as  shown  in  the  diagram 
(Fig.  147).  The  ascending  fibers  in  their  course  up  the  cord  to  the  brain 
approach  the  median  septum  thus  entering  the  gracile  fasciculus.  The 
manner  in  which  they  communicate  with  the  cells  of  cerebral  hemi- 
spheres will  be  considered  with  the  brain. 

The  lateral  funiculus  of  the  cord  consists  of  four  fasciculi,  (i)  The 
cerebellospinal  fasciculus  consists  largely  of  fibers  from  the  dorsal  nucleus 
ascending  to  the  cerebellum.  (2)  The  superficial  ventro-laieral  fasciculus 
also  contains  fibers  ascending  to  the  cerebellum.  Descending  fibers  from 
the  cerebellum,  together  with  large  numbers  of  those  connecting  the 
different  levels  of  the  cord  with  one  another,  are  found  in  the  lateral  fas- 
ciculus (3),  (4)  The  lateral  cerebrospinal  fasciculus  is  the  descending  tract 
from  the  cerebral  hemispheres  to  the  motor  cells,  being  the  path  of  volun- 
tary motor  action.  These  tracts  cross  in  the  brain  so  that  the  right  tract 
of  the  cord  is  connected  with  the  left  hemisphere  and  vice  versa. 

The  ventral  funiculus  includes  two  fasciculi.  The  ventral  fasciculus 
consists  chiefly  of  fibers  connecting  the  lateral  halves  of  the  cord  and  its 
different  levels  with  one  another.  The  small  ventral  cerebrospinal  fascic- 
ulus contains  descending  fibers  from  the  hemispheres,  most  of  which  cross 
through  the  white  commissure  to  connect  with  motor  cells  on  the  opposite 
side  of  the  cord.  Some,  like  the  fiber  shown  in  the  figure,  may  have  crossed 
at  a  higher  level  in  the  cord.  Such  fibers  as  cross  in  the  cord  are  believed 
not  to  cross  in  the  brain  so  that  all  the  motor  cells  are  thus  in  communication 
with  the  opposite  hemispheres  of  the  brain. 

The  ependyma  is  that  part  of  the  neuroglia  which  lines  the  central 


EPENDYMA. 


123 


canal.  It  appears  like  a  simple  cylindrical  epithelium  but  the  cell-Hke 
bodies  are  the  ends  of  strands  which  may  extend  clear  across  the  spinal 
cord  to  the  external  limiting  membrane.  A  nucleus  is  generally  found  in 
the  strand  near  the  central  canal;  there  may  be  others  further  away. 
Although  in  the  embryo  strands  from  the  central  canal  to  the  periphery 
are  easily  traced,  in  the  adult  these  are  largely  broken  up,  giving  rise  to 
cells  with  chief  processes  either  to  the  periphery  or  to  the  central  canal; 
if  the  radial  strand  is  lost  on  both  sides,  stellate  neurolgia  cells  result. 
These  are  shown  in  Fig  148.  (The  figure  also  shows  the  neurogha  cells 
with  concentric  fibers  characteristic  of  the  central  gray  substance,  and  a 


From  the  substantia  gelatinosa  of  a  newborn  rat. 

Xeuroelia  cell. 


Central  canal. 


Ependymal  cells.  •'' 


Xeuroglia  cell  of 
the  white  substance, 
from  a  cat  6  weeks 
old. 


Chief 
process. 


Concentric  neuroglia  cell  from  a  cat 
six  weeks  old. 


Xeuroglia  cell  of  the  gray  substance  of  the  base 
of  the  posterior  column  of  a  human  embrj'O. 


Fig.  14S.— N'eurogli.a.  Cells  from  thk  Spinal  Cord.    X  2S0. 

neuroglia  strand  with  very  numerous  delicate  processes  from  the  substantia 
gelatinosa.  These  processes  are  said  to  be  transformed  into  a  granular 
substance.  The  gelatinous  substance  contains  a  few  very  small  nerve 
cells,  a  network  of  fine  nerve  fibers  and  occasional  stellate  neurogha  cells.) 
The  ependymal  'cells'  at  birth  and  for  some  time  afterward  possess  cilia 
projecting  into  the  central  canal.  In  the  adult  they  have  disappeared. 
It  is  questionable  whether  or  not  they  arc  motile.  Single  bodies  but  not 
diplosomes  have  been  found  at  their  bases.  They  have  been  considered 
to  be  more  Hke  the  ciha  of  the  epididymis  than  like  those  of  the  trachea. 
The  neurone  theory.  Years  ago  it  was  thought  that  the  central 
nervous  system  was  a  continuous  network  of  fibers,  prolongations  of  which 


124  HISTOLOGY. 

formed  the  peripheral  nerves.  The  dorsal  root  fibers  joined  it  on  entering 
the  cord  and  the  motor  fibers  arose  from  it ;  between  the  two  was  a  dififuse 
net.  In  opposition  to  this  conception,  the  neurone  theory  set  forth  that 
the  ner\^ous  system  is  composed  of  distinct  cells,  the  neurones,  which  are 
related  to  one  another  '  by  contact  and  not  by  continuity.'  Some  even 
supposed  that  the  nerve  fibers  were  retractile  and  by  breaking  their  contact 
produced  unconsciousness.  In  recent  years  when  the  syncytial  nature 
of  many  tissues  has  been  shown  and  fibrils  have  been  found  passing  from 
cell  to  cell  in  smooth  muscle  ( ?),  neurogha,  and  some  epitheHa.  it  has  been 
reasserted  that  there  is  fibrillar  continuity  between  nen^e  cells.  The  idea 
that  the  nervous  system  is  an  intercellular  network  with  formative  or 
nutritive  cells  appended  to  it,  perhaps  comparable  with  the  elastic  network 
in  connective  tissue,  is  now  rejected.  Peripheral  fibers  are  not  found  to 
develop  by  the  anastomosis  of  chains  of  cells.  It  is  probable  but  not  certain 
that  the  connection  between  nerve  cells  is  merely  by  the  contact  of  pericel- 
lular nets  and  of  spiral  terminal  fibers  wound  about  the  cell  bodies. 


VASCULAR  TISSUE. 

The  vascular  tissues  include  the  blood  vessels  and  the  lymphatic 
vessels,  together  \\ath  the  blood  and  the  lymph. 

Blood  Vessels. 

De\'elopmext.     In  an  early  stage  the  blood  vessels  of  the  embryo 
form  a  network  in  the  splanchnopleure.     In  mammals,  as  in  the  chick 

(Fig.  20,  p.  21),  the  portion  of  the 
^  net  nearest  the  median  line  forms,  on 

gj^    /  J  ^„ either  side  of  the  body,  a  longitudi- 

^::f^^>^!^0^^  {■       ■        J  nal   vessel,  the    dorsal   aorta.     The 


br'f^^^^''^^®/    -^^^  '^  -       ^  part  of  the   net   folded   under    the 

\    '^    ^4' -  '^'^     f       ^- "  phar}-nx  constitutes  successively  the 


^w  ^i.y-~.."''\^m''  ^^ 


vitelline  veins,  the  heart,  and  the  ven- 

^''^s^.^^    ^;^^^j>.         %  /m/ aor/ag  continuous  in  front  of  the 

.^''^''' -■-.-.--,   \       ^       pharynx  with  the  dorsal  aortae.    The 

~   "     '"     ''-■'-&>  heart  first   appears  as  two  dilated 

^■^-  ''*9-  vessels,  one  on  either  side,  which  are 

Blood  vessels  from  a  rabbit  embryo  of  13  days, 

developing  as  endothelial  sprouts  (em  from       parts  of  the  general  nctwork.     Thev 

pre-existing  vessels  (b.v.j;  b.C,  blood  corpus-  r  a  J 

cie  within  a  vessel.  ^rc  brought  together  in  the  median 

line  under  the  phar}Tix  and  fuse. 
At  first  the  heart  pulsates  irregularly,  but  with  the  establishment  of  the 
circulation,  its  beats  become  rhythmical.     The  blood  flows  from  the  net 


SIXUSOIDS. 


12: 


through  the  veins  to  the  heart,  and  thence  through  the  arteries  back  to 
the  net.  All  of  the  future  vessels  of  the  body  are  believed  to  be  offshoots 
from  the  endothehal  tubes  just  described.  They  grow  out,  as  sho-un  in 
Fig.  149,  through  the  mesench}Tna  with  which  they  are  inseparably 
connected.  The  sprouts  are  at  first  sohd  but  soon  become  hoUow  except 
at  the  growing  tips.  They  may  encounter  similar  offshoots  from  the 
same  vessel  or  from  other  vessels  and  fuse  with  them.  Through  the 
anastomosis  of  such  sprouts,  networks  of  vessels  of  small  cahber  are 
produced  which  have  been  divided  into  two  t}^es,  the  sinusoid  and  capillary 
types. 

Sinusoids  are  formed  as  branches  or  subdivisions  of  a  single  vessel. 
A  vein  passing  near  a  developing  epithehal  organ  may  send  out  branches 
over  its  surface,  and  if  the  organ  itself  is  a  ramifying  structure  its  sub- 
divisions may  be  nearly  enveloped  by  these  venous  branches.     The  Hver 


VCL 


Int 


V  Ar 


Fig.  150. — Diagram  Showing  ox  the  Left  the  Liver  and  its  Sinusoids  ;  on  the  Right  the 

Pancreas  and  its  Capillaries. 

The  connective  tissue  is  represented  by  dots.     Ap.,Arter>";   Int.,  intestine ;  V.,  veins  ; 

V.  C.  I.,  vena  cava  inferior;  V.  P.,  portal  vein. 


is  related  in  this  way  to  the  vitelline  veins  (in  which  the  umbilical  veins 
later  come  to  empty).  In  the  left  portion  of  the  diagram,  Fig.  150,  the 
liver  is  shovvTi  in  hea\y  black  as  a  branching  outgrowth  of  the  intestine. 
The  portal  vein  (V.  P.),  which  is  a  persistent  part  of  the  \dteUine  veins, 
forms  a  net  of  small  branches,  the  endotheHum  of  which  is  quite  closely 
appHed  to  the  hepatic  tissue.  A  thin  but  important  layer  of  connective 
tissue  intervenes,  which  could  not  be  shown  in  the  figure  without  great 
exaggeration.  The  subdivisions  of  the  portal  (vitelline)  vein  are  the 
sinusoids  and  they  come  together  to  join  the  inferior  vena  cava,  this  part 
of  which  is  also  persistent  vitelline  vein.  A  relatively  small  hepatic  artery 
later  supphes  the  connective  tissue  around  the  ducts  of  the  liver,  but  the 
essential  vascular  system  of  tl^e  liver  is  a  single  large  vein  which  has  been 
resolved  into  a  net  of  sinusoids.  In  the  human  adult,  this  is  perhaps  the 
only  instance  of  sinusoidal  circulation.  In  the  embryo  the  mesonephros 
(a  renal  organ  of  large  size)  is  suppHed  by  sinusoids  derived  from  the 


126  HISTOLOGY, 

posterior  cardinal  veins;  the  musculature  of  the  heart  grows  into  the 
cavity  of  the  ventricle  in  plates  and  columns  covered  with  endothelium 
(Fig.  1 60),  thus  producing  a  net  of  vascular  spaces  or  sinusoids.  Although 
the  sinusoidal  circulation  persists  in  these  organs  in  lower  vertebrates, 
such  as  the  frog,  it  is  not  retained  in  man.  The  sinusoids  of  the  heart  are 
reduced  to  shallow  spaces  between  the  columns  of  muscle  seen  on  its  inner 
surface,  and  those  of  the  mesonephros  disappear  with  the  transformation 
of  that  orga.n  into  the  epididymis  and  epoophoron  in  the  male  and  female 
respectively.  Thick  walled  subdivisions  which  may  occur  in  the  course 
of  a  vessel  are  not  sinusoids.  The  latter  have  essentially  the  structure  of 
broad  capillaries,  from  which  they  differ  in  that  they  arise  from  a  single 
vessel.     They  are  therefore  wholly  venous  or  wholly  arterial. 

Capillary  circulation  arises  by  the  union  of  vascular  outgrowths 
from  two  vessels,  the  blood  in  which  flows  in  more  or  less  opposite  directions, 
in  other  words,  from  an  artery  and  a  vein.  The  vessels  to  the  lungs  are  at 
first  a  slender  blind  branch  from  a  part  of  the  aorta,  and  another  blind 
outgrowth  from  the  left  atrium  [auricle]  of  the  heart.  These  extend 
through  a  column  of  mesenchyma  to  the  epithelial  ramifications  of  the 
lung,  over  which  they  branch  and  become  united.  The  blood  flows  to 
the  lung  through  the  pulmonary  artery,  passes  into  capillaries  and  returns 
to  the  heart  through  a  vein.  A  similar  circulation  is  shown  in  the  diagram, 
Fig.  150.  It  is  essentially  an  arttrio- venous  circulation.  From  their 
mode  of  development,  capillaries  have  more  connective  tissue  around  them 
than  the  sinusoids. 

A  glomerulus  is  a  round  encapsulated  knot  of  small  subdivisions  of 
an  artery  which  reunite  before  leaving  the  capsule,  and  soon  after  form 
capillaries.  Glomeruh  occur  in  the  kidney  and  mesonephros.  They 
are  probably  to  be  regarded  as  encapsulated  capillaries  rather  than  as 
sinusoids. 

All  the  blood  vessels  of  the  young  embryo,  including  the  aorta  and  the 
heart,  are  merely  endothelial  tubes.  Capillaries  and  certain  sinusoids 
retain  this  structure  in  the  adult,  but  the  larger  vessels  have  thick  walls 
formed  by  transformation  of  the  surrounding  mesenchyma.  The  wall  of  the 
larger  vessels  consists  of  three  coats  or  layers ;  the  tunica  inlima,  which  is  the 
endothehum  with  a  thin  layer  of  elastic  connective  tissue;  the  tunica  media, 
which  is  chiefly  smooth  muscle  with  elastic  substance  intermingled;  and 
the  tunica  externa  [adventitial  which  is  a  dense  layer  of  elastic  connective 
tissue  sometimes  containing  muscle.  In  the  heart  the  intima  is  called 
endocardium;  the  media,  myocardium;  and  the  externa,  which  there  is 
covered  with  the  pericardial  mesothelium,  is  the  epicardium.  Capillaries, 
arteries,  veins,  and  the  heart  will  be  described  in  order. 


CAPILLARIES. 


12' 


Fig.  151. — Capillary  from  the  Tail 
OF  A  Tadpole.  Silver  Xitrate 
Preparation.    (After  Koelliker.) 


Capillaries  are  endothelial  tubes  of  vamng  diameter,  the  smallest 
being  so  narrow  that  the  blood  corpuscles  are  distorted  in  passing  through 
them  in  single  file.  Their  walls  are  composed  of  elongated,  very  flat  cells 
with  irregularly  wa^y  margins  as  shown  in  Fig,  151,  from  a  silver  nitrate 
preparation.  Between  the  cells  the  corpuscles,  both  red  and  white,  may 
make  their  way  out  of  the  vessel.  There 
are  no  preformed  openings  for  this  purpose, 
and  the  endothehal  cells  come  together  after 
the  corpuscles  have  passed  out.  Two  cells 
form  the  circumference  of  small  capiUaries, 
4.5  to  7  jJ-  in  diameter,  and  three  or  four 
cells  bound  the  larger  ones  of  8  to  13  fJ-. 
Nerv^es  end  in  contact  with  them  and  it  is 
possible  for  the  endothehal  cells  to  contract. 
The  bulging  of  their  nuclei  into  the  lumen 

of  the  vessel,  often  seen  in  specimens  of  capillaries  and  of  larger  vessels, 
is  probably  an  artificial  appearance.  The  lining  in  life  is  thought  to  be 
smooth.  Certain  endothehal  cells  are  said  to  be  phagocytic,  devouring 
objects  which  float  in  the  blood,  and  some  endothehal  cells  have  been 
described  as  becoming  detached  and  entering  into  the  circulation.  Small 
capiUaries  divide  without  decrease  in  cahber,  and  by  anastomosis  with 

neighboring  capillaries 
they  form  networks  differ- 
ing ^adely  in  the  size  of 
the  meshes.  The  closest 
meshes  occur  in  the  secre- 
tory organs  and  in  the 
lungs  and  mucous  mem- 
branes; the  widest  are 
in  muscles,  the  serous 
membranes  and  the  sense 
organs.  The  close  net- 
works consist  of  capil- 
laries of  large  cahber;  and 
those  with  wide  meshes 
are  formed  of  more  slen- 
der vessels.  Thus  the  blood  supply  of  glandular  organs  is  particularly 
abundant.     The  sinusoids  of  the  Hver  are  close  meshed  and  large. 

Arteries,  in  approaching  their  terminal  branches,  become  small 
{arterioles)  and  as  'precapillary  vessels'  pass  without  line  of  demarcation 
into   capiUaries.     The   smaUest  arteries   are  endothehal  tubes   encircled 


-t] 


H^ 


4% 
3 


aA- 


n 


A}' 


^. 


^'~) 


f^' 


% 


Fig.  152. — S-M.\LL  Arteries  of  M.ax. 
Nuclei  of  endothelial  cells  ;   m,  nuclei  of  circular  muscle  fibers, 
at   m'  seen  in  optical  cross  section  ;    a,  nuclei  of  connective 
tissue.     In  A,  since  the  endothelium  is  out  of  focus,  its  nuclei 
are  not  seen.    X  240. 


128  HISTOLOGY. 

by  occasional  smooth  muscle  fibers.  In  Fig.  152,  C,  the  oval  nuclei  of 
the  endothehum  are  seen  to  be  elongated  parallel  with  the  course  of  the 
vessel.  As  is  usually  the  case,  the  walls  of  the  endothehal  cells  are  not 
visible.  The  rod  shaped  nuclei  of  the  muscle  fibers  are  at  right  angles 
with  the  axis  of  the  vessel.  In  the  somewhat  larger  artery,  B,  the  muscle 
fibers  form  a  single  but  continuous  layer,  the  media,  outside  of  which  the 
connective  tissue  is  compressed  to  make  the  externa.  Its  meshes  tend 
to  be  parallel  with  the  vessel.  The  walls  of  such  an  artery  are  so  thick 
that  it  is  possible  to  focus  on  the  layers  separately;  thus  in  A,  the  endothe- 
lium which  with  a  deHcate  elastic  membrane  beneath  it  constitutes  the 
intima,  is  not  seen,  being  out  of  focus.  The  nuclei  of  the  media  and  ex- 
terna are  evident. 

The  structure  of  the  larger  arteries  is  illustrated  by  the  cross  section, 
Fig.  154.  The  intima  consists  of  endothelium  resting  on  a  layer  of  con- 
nective tissue  containing  flattened  cells  and  a  network  of  fine  elastic  fibers. 


Indentations  made  bv  smooth  muscle  fibers. 


Fig.  153.— Endothelium  of  the  Mesenteric  Artery  of  a  Rabbit.    Surface  View.    X  260. 

The  meshes  of  the  fibrous  and  elastic  tissue  are  elongated  lengthwise 
of  the  vessel  and  on  surface  view  they  present  a  longitudinally  striped 
appearance.  Toward  the  media,  the  intima  contains  a  conspicuous  inner 
elastic  membrane  which  is  fenestrated  and  usually  thrown  into  longitudinal 
folds.  (Fenestrated  membranes  have  been  described  on  page  42.)  In 
the  smaller  arteries  (those  under  2.8  mm.  in  diameter)  the  endothelium 
rests  directly  upon  the  inner  elastic  membrane;  and  in  such  large  ones 
as  the  external  ihacs,  the  principal  branches  of  the  abdominal  aorta,  and 
the  uterine  arteries  in  young  persons,  the  subendothelial  connective 
tissue  is  said  to  be  lacking.  The  inner  elastic  layer  is  very  thick  in  the 
larger  arteries  of  the  brain,  and  may  be  double. 

In  the  media  the  number  of  layers  of  circular  smooth  muscle  fibers 
increases  from  the  precapillary  vessels  which  have  but  one,  to  large  arteries 
like  the  brachial  which  have  many.  Sometimes  the  media  near  the  intima 
contains  a  few  longitudinal  fibers;    these  have  been  reported  in  the  sub- 


CAPILLARIES. 


129 


clavian,  splenic,  renal,  and  dorsaKs  penis  arteries,  and  in  the  umbilical 
arteries  they  form  a  considerable  inner  layer.  They  are  said  to  occur 
especially  near  the  places  of  branching.  Between  the  circular  muscles 
there  is  a  varying  amount  of  connective  tissue  with  wide  meshed  nets  of 
elastic  fibers.  The  proportion  between  the  muscle  and  elastic  substance 
varies  greatly.  In  the  aorta  and  pulmonary  arteries  the  elastic  tissue 
far  surpasses  the  muscular,  and  it  predominates  also  in  the  carotid,  axillary 
and  common  ihac  arteries.  Muscular  tissue  is  ascendant  in  the  distal 
arteries.     The  former  group  of  vessels  contains  the  conducting  arteries, 


Endotheliup-' 
Internal 
elastic 


2-    V 


membrane  -      '  y-x Intima. 


Elastic 
fibers     " 


Media. 


External 

elastic 
membrane. 


I 


A 


^<Sa**.5?^---'- 


^  "^gi^^cIl^L     Smooth 
%:->£o'-'''''         ^^    muscle 


>5p-  ".,  /         fibers 


Fig.  154..— Portion  of  a  Cross  Section  of  the  Brachi.\l  Artery  of  Man.     x  100. 

which  always  remain  freely  open;  the  latter  are  distributmg  arteries 
which  by  changing  their  cahber  control  the  blood  supply  in  their  areas 
of  distribution.  After  death  these  vessels  contract,  the  muscle  nuclei 
becoming  spirally  twisted,  and  the  intima  thrown  into  longitudinal  folds. 
The  blood  is  forced  on  into  the  capillaries  and  veins.  Then  as  the  rigidity 
of  the  mascles  passes  off,  the  elastic  tissue  distends  the  vessel  which  remains 
comparatively  empty  of  blood;  for  this  reason  the  ancients  supposed  that 
arteries  contained  air.  The  umbiHcal  arteries  are  exceptionally  deficient 
m  elastic  tissue  and  remain  contracted,  which  aids  in  preventing  haemor- 
rhage when  the  umbihcal  cord  is  ruptured  at  birth. 
9 


I30 


HISTOLOGY. 


Endolheliuin. 


Connective 
tissue. 


Smooth 
muscle  fibers. 


Elastic  fibers. 


The  externa  consists  of  connective  tissue,  which  is  denser  and  contains 
more  elastic  fibers  in  its  inner  portion.  A  prominent  layer  of  elastic  tissue 
near  the  media  is  called  the  outer  elastic  membrane,  and  is  especially  well 
developed  in  the  carotid,  brachial,  femoral,  coehac,  and  mesenteric  arteries. 
It  is  absent  from  the  basilar  artery  and  most  of  those  within  the  skull. 
Sometimes  the  externa  contains  scattered  bundles  of  longitudinal  muscle. 
In  the  larger  vessels  it  contains  small  nutrient    blood   vessels,  the  vasa 

vasorum.  These  may 
penetrate  the  outer 
part  of  the  media. 
Lymphatic  vessels 
often  accompany  the 
blood  vessels  and  have 
branches  in  the  ex- 
terna. Their  deeper 
penetration  is  doubt- 
ful, although  they 
have  been  reported  in 
the  intima  of  certain 
large  vessels.  Sen- 
sory nerves  may  ter- 
minate in  the  externa 
with  free  endings  or 
in  lamellar  corpus- 
cles, the  latter  being 
numerous  in  the  ab- 
dominal aorta;  free 
sensory  endings  are 
also  found  in  the  in- 
tima. The  vaso-motor 
nervts  are  non-medul- 
1  a  t  e  d  sympathetic 
fibers  which  form  plex- 
uses in  the  media  and  terminate  in  contact  with  the  muscle  fibers. 
These  plexuses  are  said  not  to  contain  ganghon  cells. 

The  largest  arteries,  the  pulmonary  and  the  aorta  (Fig.  155),  have 
a  broad  intima  which  increases  in  thickness  with  age.  It  consists  of  an 
endotheUum  of  cells  less  elongated  than  those  of  smaller  arteries,  resting 
on  fibrillar  connective  tissue  with  flattened  round  or  stellate  cells.  Its 
elastic  fibers  are  broader  toward  the  media,  but  there  is  no  distinct  inner 
elastic  membrane.     The  media  consists  of  very  many  concentric  elastic 


Elastic  fibers. 


Connective 
tissue. 


\ 


5>^'^'»^--v.~!3-: 


Vi<---  155.  — From  a  Cf<oss  Section  of  the  Thoracic  Aori  a 
or  Man.     x  ioo. 


VEINS. 


131 


lamina  connected  with  one  another  across  the  muscle  layers  which  lie 
between  them,  by  elastic  bands.  The  muscle  fibers  of  the  inner  portion 
have  been  described  as  short,  broad  and  flattened  elements  joined  to  one 
another  so  that  they  resemble  cardiac  muscle  (Fig.  156).  The  outer  muscle 
is  of  a  more  ordinary  form.  The  elastic  ele- 
ments greatly  predominate  and  on  section  the 
fresh  aorta  appears  yellow,  not  reddish  like 
smaller  vessels.  The  externa  contains  no 
outer  elastic  membrane.  It  is  relatively  and 
absolutely  thinner  than  the  externa  in  some 
medium  sized  arteries. 

Veins.  The  veins  have  thinner  walls, 
containing  less  muscle  and  less  elastic  tissue 
than  the  corresponding  arteries.     Since  the 

artery  to  any  structure  and  the  returning  vein  often  are  side  by  side,  it  is  fre- 
quently possible  to  make  such  comparisons  in  a  given  specimen.  Becauseof 
thinner  walls  the  veins  often  collapse,  or  at  least  are  not  as  circular  as 
the  arteries;  they  may  be  distended  with  blood,  and  frequently  have  a 
larger  lumen  than  the  contracted  artery.     In  many  large  veins  the  media 


Fig.  156.  —  Branched  Smooth  Mus- 
cle Cells  from  the  Thoracic 
Aorta  of  a  Child  at  Birth  (a) 
AND  AT  Four  Months  (b).  (After 
Koelliker.) 


:f^' 


Endothelium. 


f^f-Jt^   Inner  elastic 
Ifi  membrane. 


Nuclei  of 
smooth 
muscle  fibers. 


Fig.  157. — Part  of  a  Cross  Section  of  a  Vein  from  a  Human  Limb.     X  230. 
The  elastic  elements  are  drawn  very  black.    1,  Intima  ;  2,  media ;  3,  externa.     (The  middle 

of  the  3  objects  labelled  nuclei  of  smooth  muscle  is  apparently  an  elastic  fiber.) 


is  very  thin  or  even  absent,  and  the  externa,  containing  large  bundles  of 
longitudinal  muscle  fibers,  becomes  the  principal  muscular  coat. 

Venules  and  precapillary  veins  are  wider  than  the  corresponding 
arteries.    Their  endothelial  cells  are  less  elongated;  the  muscle  fibers  do 


132  HISTOLOGY. 

not  form  so  compact  a  layer  and  their  nuclei  are  oval  rather  than  rod 
shaped.  For  some  distance  from  the  capillaries  muscle  fibers  are  absent 
although  encircling  bundles  of  connective  tissue  may  be  present. 

In  the  larger  veins  (Fig.  157)  the  intima  consists  of  an  endothelium  of 
polygonal  cells  resting  on  connective  tissue  and  bounded  by  the  inner 
elastic  membrane.  The  latter  is  structureless  in  small  veins  but  is  repre- 
sented by  elastic  nets  in  the  larger  ones.  In  the  intima  of  various  veins 
occasional  obhque  or  longitudinal  muscle  fibers  have  been  found.  (These 
occur  in  the  ihac,  femoral,  saphenous  and  intestinal  veins,  the  intramus- 
cular part  of  the  uterine  veins,  and  especially  in  the  dorsal  vein  of  the  penis 
near  the  suspensory  ligament.) 

The  media  is  best  developed  in  the  veins  of  the  lower  extremity 
(especially  in  the  popliteal),  less  developed  in  those  of  the  upper  extremity, 
and  still  less  in  the  larger  veins  of  the  abdominal  cavity.  It  consists  of 
circular  muscle  fibers,  elastic  netw^orks,  and  fibrous  connective  tissue,  the 
last  being  more  abundant  than  in  the  arteries.     In  many  veins  the  media 

is  represented  only  by  connective  tis- 
I'ltima.  r-ssc:r~~t=ss33s  sue,  as  in  the  superior  vena  cava  and 
Media,  i  .^.1^=::^-^-—;:--..^  -     its  principal  tributaries;  the  veins  of 


Externa  with  longitu- )    :f^7/i^^^%-^ii.  _^    the   retina   and   of    the    bones;    and 

dinal   smooth  muscle  (      y!^  ■' '    t     ^' -   ••>''Sf  r       i  •  i       i 

fibers  cut  across.      J    «^> ,  '^1^^^'     those  of   the   pia   and    dura   mater. 

Thin  walled  veins  of  large  diameter 

Fig.  158. — Part  of  Cross  Section  of  the  .    ,    ,        i   1    i  n  1 

Human  Renal  Vein.   X  50-  m  the  dura  and  clsewhcre  are  called 

sinuses. 

The  externa  of  veins  is  their  most  highly  developed  layer.  It  con- 
sists of  crossed  bundles  of  connective  tissue,  elastic  fibers,  and  longitudinal 
smooth  muscle  which,  as  in  the  trunk  of  the  portal  vein  and  in  the  renal 
vein  (Fig.  158),  form  an  almost  complete  muscle  layer.  The  blood  and 
nerve  supply  of  veins  is  similar  to  that  of  arteries.  The  vasa  vasorum  are 
said  to  be  more  numerous  in  veins,  into  which  they  empty. 

The  valves  of  veins  are  paired  folds  of  the  intima,  each  shaped  Hke 
half  of  a  cup  attached  to  the  wall  of  the  vein  so  that  its  convex  surface 
is  toward  the  lumen.  In  longitudinal  section  they  appear  hke  the  valves 
of  the  lymphatic  vessel  shown  in  Fig.  164.  The  valves  are  generally 
found  distal  to  the  point  where  a  branch  empties  into  the  vein,  and  they 
prevent  its  blood  from  flowing  away  from  the  heart.  The  valves  do  not  oc- 
cur in  small  veins.  They  are  most  numerous  in  the  veins  of  the  extremities, 
but  appear  also  in  the  intercostal,  azygos  and  spermatic  veins.  Elsewhere 
they  are  absent.  The  endothelial  cells  on  the  surface  of  the  valve  toward 
the  lumen  of  the  vein  are  elongated  parallel  with  the  current,  but  on  the 
side  toward  the  wall  of  the  vein  they  are  transversely  placed.     Under  the 


HEART. 


133 


former  there  is  a  thick  elastic  network;    the  transverse  cells  rest  on  a 
deHcate  libered  connective  tissue. 

The  Heart.  Development.  The  heart  has  already  been  described 
as  a  median  longitudinal  vessel  beneath  the  pharynx,  formed  posteriorly 
by  the  union  of  the  vitelline  veins  and  terminating  anteriorly  in  the  two 
ventral  aortae.  Such  a  heart  from  a  rabbit  embryo  is  shown  in  Fig.  159, 
A.  It  soon  becomes  bent  Hke  a  U,  the  venous  opening  being  carried 
forward  dorsal  to  the  aortic  part  as  shown  in  B  and  C.  The  ventral  or 
aortic  hmb  of  the  U  at  the  same  time  is  carried  to  the  right  of  the  median 
plane  (C).  The  dorsal  Hmb  is  divided  into  two  parts  by  an  encircHng 
constriction,  the  coronary  sulats  {s.  c).     Its  thick  walled  portion  ventral 


-pv  vt' 


D 


Fig.  159. — Embryonic  Hearts. 
A  and  B,  From  rabbits  9  days  after  coitus  ;  C,  from  a  human  embryo  of  3  (?)  weeks  ;  D  and  E,  from  a  12 
mm.  pig  (D  sectioned  on  the  left  of  the  median  septum,  and  E  on  the  right  of  it)  ;  F,  from  a  13.6  mm. 
human  embryo,  sectioned  like  E.  The  hearts  are  all  in  corresponding  positions  with  the  left  side 
toward  the  observer,  the  anterior  end  toward  the  top  of  the  page,  the  dorsal  side  to  the  right,  ao., 
Aorta;  c.  S.,  coronary  sinus;  f.  0.,  foramen  ovale;  i.  f.,  interventricular  foramen;  I.  a.,  left  atrium; 
p.  a.,  pulmonary  artery ;  p.  v.,  pulmonary  vein  ;  r.  a.,  right  atrium  ;  s.  C,  coronary  sulcus  ;  v.,  ventricle ; 
V.  b.,  bicuspid  valve ;  v.  t.,  tricuspid  valve  ;  v.  v.,  vitelline  vein  ;  v.  v.  S.,  valves  of  the  venous  sinus. 

to  the  sulcus  is  to  form  the  ventricles  of  the  heart;  the  thin  walled  dorsal 
portion  becomes  the  atria  [auricles].  In  the  human  embryo  of  three  weeks 
(C)  the  atria  are  represented  by  a  single  cavity  subdivided  into  right  and 
left  parts  only  by  an  external  depression  in  the  median  plane.  The  right 
portion  receives  all  the  veins  which  enter  the  heart  (the  vitelline  veins  and 
their  tributaries)  and  is  much  larger  than  the  left  portion.  The  cavities  of 
the  atria  not  only  connect  with  each  other  but  they  have  a  common  outlet 
into  the  undivided  ventricle.  From  the  ventricle  the  blood  flows  out  of 
the  heart  through  the  aortic  Hmb.  In  a  complex  manner,  described  in 
text-books  of  embryology,  a  median  septum  develops,  dividing  the  heart 
into  right  and  left  halves. 


134  HISTOLOGY. 

In  the  heart  of  a  12  mm.  pig  embryo  the  septum  has  formed  (Fig. 
159,  D)  and  has  been  exposed  by  cutting  away  most  of  the  left  atrium 
and  left  ventricle.  The  septum  between  the  atria  becomes  perforated 
as  it  develops,  so  that  in  embryonic  life  the  atria  always  communicate. 
The  perforation  in  the  septum  is  the  foramen  ovale.  (The  figure  shows  the 
blind  sprout  of  endothelium  (p.v.)  growing  from  the  left  atrium  to  form 
the  pulmonary  veins.)  Between  the  left  atrium  and  ventricle  the  median 
septum  forms  a  flap-like  fold;  this  and  a  similar  fold  from  the  outer  wall 
of  the  heart  constitute  the  bicuspid  valve  [mitral].  The  median  septum 
between  the  ventricles  is  never  complete.  It  leaves  an  interventricular 
foramen  through  which  blood  passes  to  the  root  of  the  aorta,  which  is 
shown  in  E,  a  section  of  the  same  heart  made  on  the  right  of  the  median 
septum.  The  pulmonary  artery  and  the  part  of  the  aorta  near  the  heart, 
develop  first  as  a  single  vessel;  they  become  separated  from  one  another 
by  the  formation  of  a  partition  across  its  lumen.  As  long  as  the  dividing 
wall  is  incomplete,  the  blood  from  either  ventricle  may  pass  out  through 
either  artery  as  shown  in  E.  In  the  more  advanced  human  embryo,  F, 
the  partition  between  the  aorta  and  pulmonary  arteries  has  extended 
so  that  it  joins  the  interventricular  septum,  and  causes  the  interventricular 
foramen  to  open  into  the  root  of  the  aorta  only  (s). 

The  figures  E  and  F  further  show  that  the  veins  which  empty  into 
the  right  auricle  unite  to  form  the  venous  sinus  just  before  terminating. 
The  outlet  is  guarded  by  a  valve  with  right  and  left  flaps.  The  left  is 
said  to  assist  in  the  closure  of  the  foramen  ovale,  which  occurs  at  birth, 
and  leads  to  the  formation  of  the  fossa  ovalis  of  the  adult.  The  right  flap 
of  the  venous  sinus  forms  the  valve  of  the  vena  cava  [Eustachian  valve] 
and  the  valve  of  the  coronary  sinus  [Thebesian  valve].  The  coronary 
sinus,  Fig.  159,  F,  c.s.  is  the  persistent  terminal  portion  of  a  vein  which 
conveyed  the  blood  from  the  left  side  of  the  embryo  to  the  right  atrium. 
Most  of  its  branches  are  lost  by  anastomosis  with  other  vessels  so  that  in 
the  human  adult  its  territory  is  hmited  to  the  heart  itself.  It  is  found  in 
the  coronary  sulcus.  Between  the  right  auricle  and  ventricle  is  the  tri- 
cuspid valve,  similar  to  the  bicuspid  in  its  development.  These  valves 
are  seen  in  section  in  Fig.  160. 

Embryologically  the  heart  is  composed  of  three  layers,  the  endothelium, 
mesenchyma,  and  mesotheliuni.  The  endothelium  is  continuous  with  that 
which  lines  the  blood  vessels.  The  mesenchyma  which  surrounds  it, 
becomes  in  part  difi'erentiated  into  connective  tissue  which  with  the 
endothehum  makes  the  endocardium.  In  part  it  forms  cardiac  muscle,  the 
myocardium,  together  with  the  tendinous  rings  {annuli  fibrosi)  between  the 
atria  and  ventricles.     As  fibrous  connective  tissue  it  extends  into  the  valves. 


HEART. 


135 


and  in  looser  form  it  unites  with  the  mesothehum  to  make  the  epicardium. 
The  epicardium  or  visceral  pericardium  is  continuous  with  the  parietal 
pericardium  in  such  a  way  that  the  two  layers  form  a  closed  sac  which 
envelops  all  of  the  heart  except  its  base,  where  the  large  vessels  enter  and 
leave  it.  The  pericardial  cavity  within  this  sac  was  originally  continuous 
with  the  peritonaeal  cavity,  and  in  the  adult  the  walls  of  these  subdivisions 
of  the  coelom  have  essentially  the  same  structure.  It  contains  the  serous 
pericardial  fluid. 

Adult  structure  of  the  heart. 
The  endocardium  is  a  connective 
tissue  layer  covered  with  an  en- 
dothehum  composed  of  irregu- 
larly polygonal  cells.  It  contains 
some  smooth  muscle  fibers,  and 
elastic  networks  which,  in  the 
atria  especially,  form  fenestrated 
membranes.  In  the  deeper  part 
of  the  endocardium,  partially 
developed  cardiac  muscle  fibers 
occur  in  some  maramals,  but 
rarely  in  the  human  adult.  Such 
muscle  fibers,  characterized  by 
containing  only  a  peripheral  ring 
of  banded  fibrils,  are  called 
'Purkinje's  fibers'.  They  may 
be  transformed  into  typical  car- 
diac muscle.  The  valves  of 
the  heart  are  essentially  folds 
of  endocardium  containing 
dense  fibro-elastic  tissue  con- 
tinuous with  the  annuli  fibrosi. 

In  the  atrioventricular  valves  there  are  smooth  muscle  fibers,  most  abundant 
near  the  attached  borders;  and  some  blood  vessels.  The  semilunar  valves 
of  the  pulmonary  artery  and  aorta  consist  of  connective  tissue  which  is 
denser  and  more  elastic  on  the  side  toward  the  ventricles,  and  particularly 
at  the  periphery  and  nodules  of  the  valves.  The  nodules  are  thickenings 
in  the  center  of  the  circumference  of  each  segment  of  the  valve,  which 
perfect  their  approximation  when  closed.  The  endocardium  contains 
free  sensory  nerve  endings,  associated  with  modified  connective  tissue  cells, 
and  undoubtedly  motor  nerves  to  its  few  muscle  fibers.  Lymphatic 
vessels  have  been  described  in  it,  together  with  the  terminal  capillaries 


Fig.  160. — Section  of  the  IIeari  shown  in  Fig 


ca.,  Capillaries  ;  en.,  endothelium  ;  I.  a.,  left  atrium  ;  I.  v., 
left  ventricle  ;  mes.,  mesothelium  (of  the  epicardium, 
or  visceral  pericardium) ;  p  c.,  pericardial  cavity;  p.p., 
parietal  pericardium  ;  r.  a.,  right  atrium  ;  r.  v.,  right 
ventricle;  si.,  sinusoids  ;  v. b.,  bicuspid  valve;  v.  t., 
tricuspid  valve  ;  v.  v.  S.,  valves  of  the  venous  sinus. 


136  HISTOLOGY. 

of  the  epicardial  blood  vessels.  The  capillaries  of  the  heart  are  derived 
from  venous  outgrowths  of  the  coronary  sinus  which  unite  in  the  epicar- 
dium  with  arterial  outgrowths  from  the  root  of  the  aorta.  The  branches 
of  these  vessels  invade  the  myocardium  where  they  form  abundant  capil- 
lary networks  and  finally  reach  the  endocardium.  Some  of  them,  especially 
in  the  right  atrium,  empty  into  the  cavities  of  the  heart  as  small  veins, 
the  venae  minimae  [of  Thebesius].  Since  under  certain  conditions  the 
blood  may  flow  from  the  heart  cavity  to  the  myocardium  through  these 
vessels,  they  are  of  considerable  importance.  Their  embryological  history 
is  unkno\\Ti,  so  that  nothing  can  be  said  concerning  their  possible  relation 
to  the  sinusoids. 

The  myocardium  consists  of  cardiac  muscle,  the  structure  of  which 
has  been  described  on  pages  81-85,  together  with  intervening  connective 
tissue,  poor  in  elastic  elements  but  containing  many  capillaries,  motor 
nerve  fibers,  and  tissue  spaces.  Some  lymphatic  vessels  pass  through  it. 
The  musculature  of  the  atria  is  not  completely  separated  from  that  of  the 
ventricles;  there  is  an  uninterrupted  portion  in  the  median  septum.  An 
outer  obUque  layer  of  muscle  covers  both  atria  extending  from  one  to  the 
other.  Each  has  a  separate  inner  layer  of  longitudinal  bundles,  which, 
as  found  in  the  prominent  ridges  seen  in  the  interior  of  the  right  atrium, 
are  called  pectinate  muscles.  There  are  similar  but  less  prominent  struc- 
tures in  parts  of  the  left  atrium.  Besides  these  two  layers,  more  or  less 
definite,  there  are  irregularly  placed  cardiac  muscle  fibers,  and  some  which 
extend  over  the  terminal  parts  of  the  large  veins.  The  annuH  fibrosi  serve 
for  the  attachment  of  the  ventricular  muscles.  The  right  annulus  is  larger 
than  the  left.  Similar  bands  of  fibrous  tissue  surround  the  openings  of  the 
arteries.  The  complex  muscle  layers  of  the  ventricle  may  be  separated 
by  maceration  into  bands  which  arise  in  the  annuh,  wind  spirally  around 
the  heart,  and  terminate  in  the  opposite  ventricle.  The  deeper  layers 
pass  through  the  septum  and  are  arranged  in  8  or  S  shaped  figures.  Mus- 
cular elevations  projecting  into  the  ventricles  are  called  trabeculae  carneae 
if  colunmar,  or  papillary  muscles,  if  conical.  The  latter  may  be  connected 
with  the  margins  of  the  cuspid  valves  by  fibrous  prolongations,  mostly 
non-muscular,  named  the  chordae  tendineae.  These  structures  represent 
the  trabecular  framework  of  the  embryonic  heart. 

The  epicardium  consists  of  the  single  layered,  very  flat  mesothelium 
and  the  underlying  layer  of  connective  tissue,  which  contains  groups  of 
fat  cells.  Its  elastic  fibers  are  continuous  with  those  in  the  externa  of 
the  large  veins,  but  they  cannot  be  traced  beyond  the  roots  of  the  aorta 
and  pulmonary  artery.  The  epicardium  contains  lympathic  vessels,  the 
main  branches  of  the  coronary  blood  vessels,  and  important  nerves. 


LYMPHATIC   VESSELS.  I37 

The  nerves  to  the  heart  are  the  cardiac  nerves  from  the  cervical 
sympathetic  gangha,  and  certain  branches  of  the  vagus.  Together  these 
form  the  cardiac  plexus  with  the  associated  cardiac  ganglion  [of  Wrisberg] 
at  the  base  of  the  heart.  Their  fibers  extend  in  plexuses  containing 
groups  of  cell  bodies,  over  the  dorsal  walls  of  the  atria,  along  the  coronary 
sulcus,  and  over  the  ventricles  where,  however,  cell  bodies  are  less  numerous. 
They  lie  in  the  epicardium  but  extend  into  the  myocardium  and  appear 
as  bundles  of  non-medullated  fibers.  A  few  medullated  fibers,  supposed 
to  belong  chiefly  with  sensory  nerves,  are  found  with  them.  Free  sensory 
endings,  comparable  with  those  in  tendon,  are  numerous  in  the  epicardium 
and  occur  in  the  connective  tissue  of  the  other  layers.  They  include 
vagus  fibers,  which  also  terminate  in  baskets  around  the  cell  bodies 
in  the  plexuses,  but  none  are  believed  to  pass  directly  to  motor  endings. 
The  motor  terminations  belong  with  ganghon  cells  in  or  near  the  heart. 
Fibers  from  the  cervical  sympathetic  ganglia  may  end  in  pericellular 
-baskets  like  the  vagus  fibers,  or  may  pass  directly  to  the  muscles.  Their 
exact  termination  is  not  known. 

Lymphatic  Vessels. 

The  lymphatic  vessels  are  widely  distributed  through  the  body  and 
physiologically  they  are  perhaps  quite  as  important  as  the  blood  vessels. 
They  are  however  far  less  conspicuous.  For  this  reason  they  are  often 
neglected  by  the  student,  who  with  some  study  should  be  able  to  find 
them  in  a  large  proportion  of  the  specimens  examined.  In  a  rabbit  embryo 
of  14  days  and  i8  hours,  Fig.  i6i,  the  lymphatic  system  consists  of  several 
spaces  in  close  relation  with  the  veins,  lined  with  endothelium  like  that 
of  the  blood  vessels.  The  largest  sac  half  encircles  the  internal  jugular 
vein  and  sends  a  considerable  branch  into  the  deep  connective  tissue  of 
the  neck.  Another  large  lymph  space  is  near  the  renal  veins;  smaller 
ones  are  with  the  mesenteric  vessels,  the  azygos,  and  the  external  mammary 
veins.  An  examination  of  younger  embryos  indicates  that  these  lymphatic 
vessels  are  detached  branches  of  the  adjacent  veins.  They  are  closed 
endothelial  tubes  which  send  out  ramifying  branches  into  the  subcutaneous 
and  other  connective  tissue,  where  they  anastomose  with  one  another  or 
end  bhndly.  They  do  not  anastomose  with  the  blood  vessels,  which  they 
resemble,  except  for  thinner  walls  and  larger  lumen.  All  of  the  lymphatic 
structures  in  the  rabbit  of  14  days  become  connected  with  each  other  and 
with  similar  new  lymphatic  vessels  so  as  to  form  a  system  which  empties 
into  the  veins  at  two  points,  namely,  into  the  subclavian  veins  near  the 
internal  jugulars,  on  either  side  of  the  body.  These  openings  have  been 
described  as  persistent  original  connections  of  the  lymphatic  vessels  with 


138 


HISTOLOGY. 


the  veins,  but  they  cannot  be  detected  in  the  rabbit  figured;  they  may  be 
formed  later  when  the  lymphatic  system  is  essentially  complete.  In  the 
adult  the  lymphatic  vessels  from  the  legs  follow  the  femoral,  hypogastric 

and  common  ihac  vessels 
to  the  aorta,  in  front  of 
which  they  form  a  net- 
work. Here  they  are 
joined  by  lymphatics  from 
the  viscera,  notably  from 
the  in  testines.  The  latter 
vessels  were  called  lac- 
teals  from  their  milky 
appearance  when  filled 
with  fat  obtained  from 
the  ahmentary  canal. 
The  net  is  continued  into 
the  thorax  as  one  large 
vessel,  the  thoracic  duct, 
which  may  or  may  not  be 
enlarged  at  its  origin, 
forming  the  cistern  a  chyli. 
The  thoracic  duct 
receives  intercostal 
branches;  in  places  it 
may  be  irregularly  re- 
solved into  several  small 
vessels  which  reunite. 
Near  its  termination  in 
the  left  subclavian  vein 
the  thoracic  duct  receives 
subclavian  and  jugular 
trunks  from  the  left  arm 
and  left  side  of  the  head 
On    the 


Fig. 


Rabbi 


i6i.— Lymphatic  Vessels  and  Veins  in 
14  Days,  18  Hours;  14.5  mm.  X  n-S- 
The  lymphatics  are  heavily  shaded,  x  being  along  the  left  vagus 
nerve  and  y  along  the  aorta.     The  subclavian  vein  is  formed  bv 
the  union  of  the  primitive  ulnar,  Pr.  Ul.,  and  external  mammary 
veins,  Ex.  M.     The  other  veins  are:     In.  J.  and  Ex.  J.,  internal     rpQnprtivplv 
and  external  jugulars  ;  Ce.,  cephalic  ;  Az.,azygos;  V.,  vitelline  ;     ^cspcLLivci^y . 

Seff=^S=,^i:?;'^ret1nfer'Ur!^'k;irFer^=i-   right  side  there  is  a  n^/./ 

An.  T..  anterior  t,bia,;Pr.Fi,  primitive  fibular  ;c.b.,  branch  to     ^^^^;,^^,-,       ^^,^     f^^.^^.^ 


connect  with  the  femoral  vein. 


by  the  union  of  vessels 
from  the  right  arm,  the  right  side  of  the  head  and  heart,  and  the 
right  lung.  Sometimes  the  thoracic  duct  bifurcates  in  the  thorax  sending 
a  branch  to  the  right  lymphatic  duct,  or  its  main  stem  may  be  on  the  right 
side.     Instead  of  a  single  opening  of  each  duct  into  the  vein,  there  may  be 


LYMPHATIC   VESSELS. 


139 


/I  V  '  h  V 


Fig   162 

Connective  tissue  trom  the  submucous  laj'er  of 

the  small   intestine  of  a   cat,   showing  one 

blood  vessel,  b.  v.;  three  lymphatic  vessels, 

I.  v.:  and  numerous  intercellular  spaces,  i.  s. 


small  intestines  from  animals  in  which  intestinal  digestion 


two  or  more.  From  its  development  the  lymphatic  system  is  a  part  of 
the  venous  system,  consisting  of  endothehal  tubes  ramifying  in  connective 
tissue,  anastomosing  with  each  other 
or  ending  bhndly.  Its  striking  char- 
acteristic is  that  it  is  wholly  afferent;  it 
is  like  a  venous  system  which  has  no 
corresponding  arteries.  The  fluid 
within  it  is  derived  from  the  intercellu- 
lar tissue  fluids. 

The  smaller  lymphatic  vessels 
may  be  studied  advantageously  in  sec- 
tions  of  the 

is  in  progress.  The  lymphatics  are  then  dilated.  They  appear  as  spaces 
in  the  connective  tissue  (Fig.  162)  which  are  sharply  defined,  thus  contrast- 
ing with  the  intercellular 
spaces.  Their  distinct  lining 
is  due  to  endothehum,  the 
nuclei  of  which  are  often 
seen.  They  have  the  struc- 
ture of  capillaries  but  are  of 
larger  size;  blood  vessels  of 
similar  caliber  have  thicker 
walls.  The  lymphatic  ves- 
sels often  appear  empty  or 
contain  a  granular  coagulum,  whereas  red  blood  corpuscles  are  to  be 
expected  in  the  blood  vessels.  A  structure  containing  many  red  corpus- 
cles may  be  safely  regarded  as  a  blood  vessel,  but  obviously  an  empty 
vessel  is  by  no  means  a  lym- 
phatic. Occasional  red  cor- 
puscles find  their  way  into 
lymphatic  vessels.  In  silver 
nitrate  preparations  (Fig. 
163)  the  lymphatic  endo- 
thelium is  seen  to  be  simi- 
lar to  that  of  the  blood 
vessels.  Valves  are  numer- 
ous even  in  small  lymphatic 
vessels.  They  are  folds  of 
endothehum  such  as  would 

result  if  the  distal  part  of   the  vessel   were   pushed  forward    into   the 
proximal  part.     The  vessels  are  often  distended  on  the  proximal  side  of 


Fig.  163. — Silver  Nitrate  Preparation  of  a  Lymphatic 
Vessel  from  a  Rabbit's  Mesentery,  Showing  the 
Boundaries  of  the  Endothelial  Cells,  and  a 
Bulging  Just  Be\"ond  a  Valve. 


Fig.  164. 
Lymphatic  vessel  from  a  section  of  a  human  bronchus,  show- 
ing a  valve,  v. ;  distal  to  the  branch ,  br.     Bundles  of  smooth 
muscle  fibers  are  seen  at  m.  f. 


I40  HISTOLOGY. 

the  valve,  as  may  be  shown  in  injected  specimens  especially.  One  of  these 
swellings  is  shown  in  Fig.  163.  The  valves  of  a  larger  lymphatic  vessel 
appear  in  Fig.  164. 

In  lymphatic  vessels  having  a  diameter  of  0.2-0.8  mm.  or  more, 
three  layers  may  be  distinguished  very  similar  to  those  of  thin  walled  veins. 
The  intima  consists  of  endotheUum  and  connective  tissue  containing  deU- 
cate  elastic  nets  with  longitudinal  meshes.  The  media  has  circular  smooth 
muscle  and  but  little  elastic  tissue.  The  externa  has  bundles  of  longitu- 
dinal muscle  fibers,  and  similarly  arranged  connective  tissue.  The  nerve 
supply  is  Hke  that  of  the  blood  vessels. 

Although  the  present  tendency,  based  upon  the  similar  results  of  several 
investigations,  is  to  make  a  sharp  distinction  between  tissue  spaces  and  lymphatic 
vessels,  it  should  be  noted  that  these  have  long  been  regarded  as  inseparable. 
Some  authorities  still  consider  that  the  lymphatic  vessels  open  freely  at  their 
distal  ends  and  blend  with  connective  tissue.  Lymphatic  vessels  have  also  been 
described  as  opening  into  the  peritoneal  cavity  and  other  parts  of  the  coelom 
through  definite  mouths  or  stomata.  The  stomata  are  thought  to  be  artificial. 
The  endothelium  remains  entirely  separate  from  mesothelium  so  far  as  is  known. 


Blood. 

Blood  consists  of  rounded  cells  entirely  separate  from  one  another 
floating  in  an  intercellular  fluid,  the  plasma.  The  plasma  also  contains 
fragments  of  cells  called  blood  plates  or  platelets,  together  with  smaller 
granular  bodies.  The  blood  cells  or  corpuscles  are  of  two  sorts,  (i)  red 
corpuscles  (erythrocytes)  which  become  charged  with  the  chemical  com- 
pound, haemoglobin,  and  which  lose  their  nuclei  as  they  become  mature; 
and  (2)  white  corpuscles  (leucocytes)  which  are  of  several  kinds,  all  of  them 
retaining  their  nuclei  and  contaming  no  haemoglobin.  The  redness  of 
blood  is  not  due  to  the  plasma,  but  is  an  optical  effect  produced  by  super- 
posed layers  of  the  haemoglobin-filled  red  corpuscles.  Thin  films  of 
blood,  like  the  individual  red  corpuscles  as  seen  fresh  under  the  micro- 
scope, are  yellowish  green.  Blood  has  a  characteristic  odor  which  has 
been  ascribed  to  volatile  fatty  acids;  it  has  an  oily  feeling  associated  with 
its  viscosity,  an  alkaline  reaction  and  a  specific  gravity  said  to  average  in 
the  adult  from  1.050  to  1.060. 

Red  corpuscles.  The  first  cells  in  the  blood  are  apparently  all  of  one 
sort,  derived  from  the  blood  islands.  They  are  large,  round  cells  with 
a  dehcate  membrane  and  a  pale  granular  protoplasmic  reticulum;  their 
relatively  large  nuclei  contain  a  chromatin  network  with  several  coarse 
chromatin  masses.     Haemoglobin  develops  in  their  protoplasm  giving  it 


RED    CORPUSCLES.  I4I 

a  refractive  homogeneous  appearance.  Stained  with  orange  G  or  eosin 
it  is  clear  and  brightly  colored,  generally  quite  unHke  any  other  portion 
of  the  specimen.  Often  the  haemoglobin  has  been  more  or  less  dissolved 
from  the  corpuscles  which  then  appear  granular  or  reticular.  Mean- 
while the  nucleus  becomes  smaller  and  so  dense  as  to  appear  a  structureless 
mass,  stained  nearly  black  with  haemotoxyhn.  This  transformation  of  the 
cells  is  sho^Ti  in  Fig.  165.  Cells  which  are  destined  to  produce  red  cor- 
puscles are  called  erythroblasts,  especially  in  the  stages  with  reticular 
nuclei.  The  later  stages  when  the  cells  are  smaller  and  have  dense  nuclei 
are  called  normoblasts.  The  nuclei  of  normoblasts  have  been  seen  to  be 
extruded  as  in  Fig.  165.  Before  they  disappear  they  may  become  mul- 
berry, dumb-bell,  or  trefoil  shaped,  (as  in  the  group  in  the  lower  left  hand 
comer  of  Fig.  174,  p.  152)  or  they  may  fragment  into  several  dark  masses. 
These  are  said  to  be  extruded  so  that  they  He  free,  outside  of  the  cell,  where 
they  are  devoured  by  phagocytes.  On  the  other  hand  it  is  beHeved  by 
some  that  extrusion  never  occurs   as  a 

normal  process,  but  that  the  nuclei  are  ^m      M^ 

dissolved  within  the  cell.     The  question  /"^h^  f'''lr\    - 

has  long  been  discussed  and  is  not  set- 
tied.     The  loss  of  the  nuclei  begins  in  \  -"-  "^  ' 

human  embryos  of  the  second  month; 
at  the  third  month  nucleated  corpuscles 
are   still  more  numerous  than  the  non-        fig.  165.— thh  development  of  red 

Corpuscles. 

nucleated.        At    birth    and    afterwards    it        a,  successive  stages  in  the  development  of 

erythroblasts,  from  a  cat  embryo  ;  b,  the 
is  unusual  to   find   nucleated    red  corpus-  extrusion  of  the  nucleus  in  cat  embryos. 

cles  in  the  circulating  blood. 

The  erythroblasts  at  first  divide  by  mitosis  in  the  blood  vessels  every- 
where. Later  they  gather  about  the  sinusoids  of  the  liver.  Apparently 
they  are  not  only  within  the  blood  vessels  but  also  outside  of  them,  in  the 
reticular  tissue  between  the  endothehum  and  the  hepatic  cells.  Red  blood 
corpuscles  both  nucleated  and  non-nucleated  are  flexible  bodies  incapable 
of  amoeboid  movement;  accordingly  they  pass  out  between  endothelial 
cells  less  readily  than  the  leucocytes.  The  emigration  of  red  corpuscles 
is  called  diapedesis.  In  fetal  Hfe  erythroblasts  multiply  not  only  in  the 
liver  but  also  in  the  spleen.  Except  in  a  few  mammals  the  spleen  does  not 
normally  retain  this  function  in  the  adult.  The  red  bone  marrow  becomes 
the  essential  permanent  location  for  the  production  of  red  corpuscles, 
and  throughout  hfe  it  contains  the  multiplying  erythroblasts.  In  certain 
important  diseases  normoblasts  leave  the  marrow  and  occur  in  the  circu- 
lating blood,  sometimes  together  with  large  forms  having  reticular  nuclei, 
and  called  megaloblasts.  The  megaloblasts  have  been  regarded  as  younger 
erythrocytes  than  the  normoblasts. 


142 


HISTOLOGY. 


With  the  loss  of  the  nuclei  the  red  corpuscles  become  smaller  and  cup 
shaped;  they  are  convex  on  one  side  and  concave  on  the  other.  ("Bell 
shaped,'  implying  a  flaring  rim,  is  a  less  descriptive  term;  'saucer  shaped,* 
signifying  that  they  are  often  shallow  cups,  has  lately  been  employed.) 
The  protoplasmic  reticulum  has  disappeared  and  the  mature  corpuscle 
has  been  said  to  be  a  drop  of  dissolved  haemoglobin  enclosed  in  a  mem- 
brane.    With  special  methods  a  granular  network  has  been  demonstrated 

in  some  apparently  homogeneous  corpus- 
cles. Others  in  the  same  preparation 
may  contain  no  reticulum.  The  network 
has  been  interpreted  as  the  remains  of  the 
original  protoplasmic  net,  and  also  as  an 
artificial  decomposition  of  haemoglobin. 
It  occurs  especially  in  the  newly  formed 
corpuscles  (seen  in  cases  of  anaemia). 
Instead  of  a  net  there  may  be  rings  or 
round  bodies  the  nature  of  which  is  not 
clear.  The  existence  of  a  membrane 
around  the  corpuscles  is  still  debated.  It  does  not  stain  distinctly, 
and  seems  to  blend  with  its  contents.  Sometimes  it  is  described  as  an 
exoplasmic,  fatty  layer.  The  osmotic  changes  in  the  corpuscles  show 
that  they  are  surrounded  by  structures  which  are  not  composed  of  haemo- 
globin, and  which  act  as  membranes. 

Cup  shaped  corpuscles  may  be  observed  circulating  in  the  omentum 
of  a  guinea  pig.     The  etherized  animal  should  be  placed  beside  the  stage 


Fig.  i66. — Red  Corpusclks,  Sketched 
WHILE  Circulating  in  the  Vessels 
OF  THE  Omentum  of  a  Guinea  Pig. 


ci#* 


B  C 


Fig.  167. — Red  Corpuscles  in  Various  Conditions. 

of  the  microscope  and  the  omentum  spread  over  the  condenser.  A  cover 
glass  is  put  directly  upon  it,  and  the  corpuscles  are  examined  with  an  oil 
immersion  lens.  Some  of  them  drawn  freehand  while  they  were  under 
observation  are  shown  in  Fig.  166.  If  a  drop  of  blood  from  the  finger  is 
spread  upon  a  slide  in  a  thin  layer  and  examined  at  once  some  cup  shaped 
forms  are  seen.  They  soon  flatten  into  biconcave  discs,  appearing  as  in 
Fig.  167,  A.    Their  thin  centers  appear  light  in  ordinary  focus,  but  become 


RED    CORPUSCLES.  143 

dark  if  the  objective  is  raised  (Fig.  167,  C).  The  biconcave  shape  is  appar- 
ent when  a  corpuscle  is  seen  on  edge  (Fig.  167,  B).  This  form  of  the  red 
corpuscles  is  still  ordinarily  described  as  normal,  since  it  is  observed  in 
freshly  drawn  blood.  The  making  of  the  thin  layer  has,  however,  sub- 
jected the  blood  to  very  unnatural  conditions.  Very  quickly  the  corpuscles 
arrange  themselves  in  'rows,  or  rouleaux  (Fig.  173),  such  as  are  not  found 
within  the  blood  vessels.  In  most  of  the  sections  which  the  student 
examines,  in  preparing  which  various  preserving  fluids  have  been  used, 
cup  shaped  corpuscles  will  be  seen  like  those  in  Fig.  167,  D,  Often  they 
will  show  irregular  contractions  and  distortions  (E).  If  the  corpuscles 
are  placed  in  a  dilute  fluid,  their  haemoglobin  is  dissolved  out  and  water 
enters  them.  They  become  mere  flattened  membranes  or  shadows  (Fig. 
167,  F).  Such  barely  visible  structures  are  sometimes  found  in  urine.  In 
dense  solutions,  or  in  ordinary  fresh  preparations  as  they  begin  to  dry, 
water  leaves  the  corpuscle,  which  shrinks,  producing  nodular,  refractive 
masses  of  haemoglobin  called  crenated  corpuscles  (Fig.  167,  G).     A  0.6  % 


1,  Haemin  crystals  and  3,  haematoidin  crystals  from  human  blood  ;  2,  crystals  of  common  salt  (X  560); 
4,  haemoglobin  crystals  from  a  dog  (X  100). 

aqueous  solution  of  common  salt  is  said  to  cause  the  least  distortion 
from  swelHng  or  shrinkage.  In  life,  corpuscles  presumably  change  their 
shape  with  variations  in  the  plasma  and  in  the  nature  of  the  haemoglobin. 
A  small  number  of  spherical  corpuscles  is  said  to  occur  normally.  When 
a  drop  of  blood  is  heated  to  excess  the  corpuscles  form  small  globules 
united  by  stalks  or  entirely  separate.  This  indicates  a  viscid  membrane, 
but  does  not  prove  the  entire  absence  of  membrane  as  has  been  asserted. 
In  strong  picric  acid  the  corpuscles  burst,  discharging  their  contents 
through  a  rent  in  a  capsule  which  may  be  largely  due  to  the  reagent. 

Haemoglobin  is  an  exceedingly  complex  chemical  substance  which 
combines  readily  with  oxygen  to  form  oxyhaemoglobin.  To  the  latter  the 
bright  color  of  arterial  blood  is  due.  Venous  blood  becomes  similarly  red 
on  exposure  to  air.  Through  the  oxyhaemoglobin,  oxygen  is  transferred 
from  the  lungs  to  the  tissues.  Haemoglobin  niay  be  dissolved  from  the 
corpuscles  by  mixing  blood  with  ether,  and  upon  evaporation  it  crystalhzes 
in  rhombic  shapes  which  vary  with  different  animals.  Those  from  the 
dog  are  shown  in  Fig.  1 68, 4;  in  man  they  are  also  chiefly  prismatic.    Haemo- 


144 


HISTOLOGY. 


9 


Leucocyte 
in  motion  ;  at  rest. 


globin  is  readily  decomposed  into  a  variety  of  substances,  some  of  which 
retain  the  iron  which  is  a  part  of  the  haemoglobin  molecule;  others  lose  it. 
Haematoidiii,  considered  identical  with  a  pigment  (bilirubin)  of  the  bile, 
is  an  iron-free  substance  occurring  either  as  yellow  or  brown  granules, 
or  as  rhombic  cn'stals.  The  crystals  (Fig.  i68,  2)  may  be  found  in  old 
blood  extravasations  within  the  body,  as  in  the  corpus  luteum  of  the  ovary. 
Haemosiderin,  which  contains  iron,  appears  as  yellowish  or  bro\^'Ti  granules 
sometimes  extremely  fine,  either  within  or  between  cells.  The  iron  may 
be  recognized  by  the  ferro-cyanide  test  which  makes  these  minute  granules 
bright  blue.  If  dry  blood  from  a  stain  is  placed  on  a  shde  with  a  crystal 
of  common  salt  the  size  of  a  pin-head,  and  both  are  dissolved  in  a  large 
drop  of  glacial  acetic  acid  which  is  then  heated  to  the  boihng  point,  a  com- 
bination of    a  haemoglobin  product  with  hydrochloric  acid  is  formed, 

called  haemin.  It  crystallizes  in 
rhombic  plates  or  prisms  of  ma- 
hogany brown  color  (Fig.  168,  i). 
Such  crystals  would  show  that  a 
suspected  stain  was  a  blood  stain, 
but  they  afford  no  indication  of 
the  species  of  animal  from  which 
it  was  derived. 

The  dimensions  of  red  cor- 
puscles are  quite  constant.  Those 
in  human  blood  average  7.5  «  in 
diameter  and  ordinarily  vary 
from  7.2  to  7.8  iJ:  They  some- 
times surpass  these  limits.  In 
biconcave  form  they  are  about  1.6  y-  thick.  The  cups  average  7  /<  in 
diameter  and  are  4  «  in  depth.  Spherical  corpuscles  are  said  to  be  5  ;>■ 
in  diameter.  The  blood  of  mammals  other  than  man  also  contains  cups 
which  become  discs.  The  latter  are  oval  in  the  camel  group  but  round  in 
all  others.  Their  average  diameters  are  less  than  in  man  (7.3  u.  in  the  dog, 
7.48  !'■  in  the  guinea-pig),  but  the  species  of  animal  cannot  satisfactorily  be 
determined  from  the  diameter  of  the  corpuscles.  It  should  be  noted  that 
the  blood  of  amphibians,  reptiles  and  birds,  in  the  adult  contains  only 
nucleated  red  corpuscles  which  are  oval  discs  more  or  less  biconvex. 
They  are  very  large  in  amphibia  (Fig.  169). 

The  number  of  red  corpuscles  in  a  cubic  milHmeter  of  human  blood 
averages  five  million  for  men,  and  four  million  five  hundred  thousand  for 
women.  By  diluting  a  small  measured  quantity  of  blood  and  spreading 
it  over  a  specially  ruled  slide,  the  corpuscles  may  be  counted,  and  the  num- 


Side  view  of 
red  corpuscles. 


Fig.  169.— Blood  CoRPfsCLES  from  a  Frog. 

4,  5,  and  6,  Surface  views  of  red  corpuscles,  6  after 

treatment  with  water.     X  600. 


RED    CORPUSCLES.  1 45 

ber  per  cubic  millimeter  calculated.     A  diminished  number  is  of  clinical 
importance. 

The  duration  of  the  Hfe  of  mature  red  corpuscles  is  unknown,  but  is 
supposed  to  be  brief.  They  may  be  devoured  intact  by  phagocytes,  but 
generally  they  first  break  into  numerous  small  granules.  These  may  be 
ingested  by  certain  leucocytes,  or  by  the  peculiar  endothehal  cells  of  the 
liver.  Their  products  are  thought  to  be  eliminated  in  part  as  bile  pig- 
ment. The  destruction  of  red  corpuscles  occurs  especially  in  the  spleen 
and  haemolymph  glands ;  to  a  less  extent  in  the  lymph  glands  and  red  bone 
marrov?.  Pigmented  cells  in  some  of  these  structures  derive  their  pig- 
ment from  destroyed  corpuscles.  Sometimes  a  'stippHng'  or  granule 
formation  occurs  within  the  corpuscle,  which  has  been  ascribed  to  degenera- 
tion of  the  haemoglobin.  The  dissolution  of  red  corpuscles  is  known 
as  haemolysis  and  follows  the  injection  of  certain  poisonous  substances 
into  the  blood.  It  occurs  in  various  diseases.  The  study  of  the  effects 
of  mixing  the  blood  of  one  species  of  animal  with  that  of  another  has  pro- 
vided a  very  perfect  means  of  distinguishing  the  species  from  which  a 
blood  stain  of  unknown  origin  may  have  been  derived.  Such  studies  are 
not  histological,  however. 

The  account  of  the  mammalian  red  corpuscles  may  be  summarized 
as  follows.     Erythroblasts  with  large  reticular  nuclei,  cell  membranes, 
and  a  protoplasmic  net,  are  the  first  blood  cells  in  the  embryo.     They 
multiply  by  mitosis  in  the  circulating  blood,  and  most  of  them  by  acquiring 
small   dense  nuclei   become  normoblasts.     Haemoglobin  has  meanwhile 
developed  in  their  protoplasm  which  loses  its  reticulum.     The  mem- 
brane is  no  longer  well  defined.     The  nucleus  after  more  or  less  fragmen- 
tation becomes  either  absorbed  or  extruded  from  the  cell,  which  thereupon 
is  cup  shaped.     The  cups  are  flexible  and  very  susceptible  to  osmotic 
changes,  swelling  or  shrinking  with  alterations  in  the  density  of  the  sur- 
rounding plasma.     They  are  destroyed  by  dissolution  or  fragmentation,    I 
and  are  often  devoured  by  phagocytic  cells.     From  them  pigments  with  or  | 
without  iron  are  developed.     The  red  corpuscles  in  the  adult  are  formed  / 
chiefly  in  the  red  bone  marrow,  and  are  destroyed  especially  in  the  spleen  / 
and  haemolymph  glands;  some  of  their  products  areeHminated  in  the  bile. ' 

White  corpuscles.  The  leucocytes  are  those  blood  cells  which  retain 
their  nuclei  and  do  not  contain  haemoglobin.  About  eight  thousand 
occur  in  a  cubic  milHmeter  of  human  blood.  If  their  number  exceeds 
ten  thousand  the  condition  is  called  leucocytosis  and  becomes  of  chnical 
importance.  There  exists,  therefore,  normally  but  one  leucocyte  for  five 
or  six  hundred  red  corpuscles.  In  the  circulating  blood  the  two  sorts  are 
said  not  to  be  evenly  mixed;  the  leucocytes  are  more  numerous  in    the 


146  HISTOLOGY. 

slower  peripheral  part  of  the  blood  stream,  near  the  endothelium.  The 
leucocytes  may  be  divided  into  three  classes  according  to  nuclear  charac- 
teristics, namely  into  lymphocytes,  large  mononuclear  leucocytes,  and  ^0/3'- 
morphonuclear  leucocytes. 

T.ympVin^ytP';  are  large  and  small.  The  ordinary  small  ones  are 
about  the  size  of  red  corpuscles,  4  to  7.5  li  in  diameter.  Large  ones  may 
double  this  diameter.  Their  protoplasm  forms  a  narrow  rim,  sometimes 
almost  imperceptible,  about  the  dense  round  nucleus  (Fig.  170,  A).  The 
chromatin  is  arranged  in  a  network  associated  with  coarse  chromatic 
masses  such  as  cause  a  characteristic  checkered  appearance.  Some  of 
the  masses  rest  against  the  nuclear  membrane.  Lymphocytes  are  capable 
of  amoeboid  motion  but  not  to  the  extent  of  the  polymorphonuclear  t}-pe. 
They  form  from  22  to  25  %  of  all  leucocytes. 

Lar^e  mononuclear  leucocytes,  sometimes  20  //.  in  diameter,  possess 
round,  oval,  sUghtly  indented,  or  crescentic  nuclei,  which  are  vesicular 
and  usually  eccentric  in  position.     Their  chromatin  occurs  in  a  few  large 

granules;  as  a  whole  the  nu- 
cleus is  clear  and  pale  (Fig, 
170,  B).  The  protoplasm, 
which  is  abundant,  usually 
lacks  coarse  granules  or  other 
I^i^^^i^^^^^^^^^^^i^^J^L.        distinctive    features.       Some- 

A,  Lymphocyte  :  B,  large  mononuclear  leucocyte;  C.  three        timeS  it  COntainS  a  fcW  neutrO- 
polymorphonuclear  neutrophiles. 

philic  granules  to  be  described 
presently.  These  cells  are  notably  phagocytic.  They  form  only 
from  I  to  3  %  of  the  leucocytes.  In  certain  respects  they  are  inter- 
mediate between  lymphocytes  and  polymorphonuclear  cells. 

Polymorphonuclear  leucocytes  are  cells  somewhat  larger  than  red 
corpuscles,  being  from  7.5  to  10  11  in  diameter.  They  are  characterized 
by  having  nuclei  with  irregular  constrictions  leading  to  an  endless  variety 
of  shapes  (Fig.  170,  C).  The  nodular  subcUvisions  may  be  connected  by 
broad  bands  or  by  slender  filaments.  It  is  said  that  in  degenerating  cells 
the  nucleus  is  divided  into  several  separate  masses.  Such  unusual  forms 
can  properly  be  called  '  pohnuclear,'  an  abbreviated  term  which  is  a  mis- 
nomer as  applied  to  the  ordinary'  cells;  'mononuclear'  as  designating  the 
preceding  t}pes  is  also  unfortunate  since  it  imphes  that  others  have  several 
nuclei.  The  irregular  shape  of  the  polymorphous  nuclei  has  been  ascribed 
to  degeneration,  comparable  with  irregularities  in  the  erythroblast  nuclei, 
and  also  to  amoeboid  changes  associated  with  those  of  the  cell  body. 
It  has  been  asserted  that  the  nuclei  become  rounded  when  the  cells  are  at 
rest.     The  latter  explanation  appears  improbable.     In  the  protoplasm  a 


WHITE    CORPUSCLES. 


147 


centrosome,  or  a  group  of  its  minute  subdivisions,  has  been  found  in  the 
concavity  of  the  nucleus.  A  deHcate  cell  membrane  has  been  described, 
but  membranes  are  usually  considered  lacking  in  all  forms  of  leucocytes. 
A  fundamental  characteristic  of  polymorphonuclear  leucocytes  is  the  devel- 
opment of  distinct  granules  in  their  protoplasm.  These  are  more  definite 
structures  than  occur  in  ordinary  protoplasm,  so  that  lymphocytes  together 
\\dth  the  large  mononuclear  cells  are  considered  non-granular.     Xot  only 


.  jpAi\jA->-i 


"'^"^  ^A 


Fig.  171. — The  Blood  Corpuscles.     (Wright's  Staix.)     (E.  F.  Faber,  from 

Da  Costa's  Clinical  Hematology-.) 

I,  Red  corpuscles.     II,  Lymphocytes  and  large  mononuclear  leucocjles.     Ill,  Neutrophiles. 

IV,  Eosinophiles.     V,  Myelocytes  (not  found  in  normal  blood).     VI,  >Iast  cells. 

do  the  granules  differ  in  size  but  also  in  staining  reaction  as  may  be  seen 
by  employing  the  'blood  stains.'  A  drop  of  blood  is  spread  thinly  on  a 
cover  glass  and  dried,  afterwards  being  stained  vA\h  a  mixture  of  acid 
and  basic  dyes.  The  details  of  nuclear  structure  are  not  preserved,  but 
the  granules  are  clearly  differentiated.  With  several  of  the  blood  stains 
the  fine  granules  stain  purple  or  hlac  and  the  coarse  ones  are  red  in  some 
cells  and  blue  in  others.  Only  one  sort  of  granule  occurs  in  a  single  cell. 
Figure  171  shows  corpuscles  from  such  a  preparation. 


148  HISTOLOGY. 

Cells  containing  coarse  blue  granules,  which  often  obscure  the  nucleus, 
are  called  mastceUs_.  (The  German  word  mast,  meaning  food,  was  applied 
to  them  because  of  supposed  nutritive  functions.)  They  form  about 
0.5  %  of  the  leucocytes  in  the  blood.  Along  the  blood  vessels,  especially  in 
the  mesentery,  mast  cells  may  be  found  in  connective  tissue  if  it  is  hardened 
in  alcohol  and  stained  with  a  basic  stain  hke  methylene  blue.  Zenker's 
fluid,  a  preservative  often  used,  destroys  these  granules.  (The  mast  cells 
of  connective  tissue  are  larger  than  those  in  the  blood,  and  generally  have 
rounded  nuclei.  They  have  been  said  to  arise  independently  of  the  "mast 
leucocytes.") 

Polymorphonuclear  cells  with  coarse  granules  which  stain  red  with 
eosin,  an  acid  stain,  are  called  eio^iiicj&^^j^ [oxyphiles,  acidophiles].  They 
form  from  2  to  4  %  of  the  leucocytes  in  the  blood,  a  proportion  greatly 
increased  in  certain  diseases.  Eosinophilic  cells  occurring  in  connective 
tissue  sometimes  have  round  nuclei.  It  is  questionable  whether  such 
forms  are  derived  from  the  eosinophiles  which  migrate  from  the  vessels. 

The  third  type  of  granular  cell,  unUke  the  other  kinds,  contains  fine 
granules,  and  these  stain  purple  or  hlac  by  taking  both  stains  to  some  extent. 
They  are  called  neutrq^Jiiles  and  form  70  to  72  %  of  the  leucocytes  in  the 
blood.  They  are  actively  amoeboid  and  are  the  principal  wandering  cells 
of  the  body,  leaving  the  blood  vessels  more  readily  than  other  forms. 

The  relation  of  the  various  leucocytes  to  one  another  has  not  yet  been 
determined.  The  first  forms  which  appear  in  embryonic  blood  have 
rounded  nuclei  and  are  perhaps  intermediate  between  lymphocytes  and 
large  mononuclear  leucocytes.  They  resemble  the  young  erythroblasts 
from  which  they  may  be  derived.  Many  authorities  consider  it  probable 
that  there  is  a  common  origin  for  all  the  blood  cells.  Like  the  red  cor- 
puscles the  leucocytes  in  the  adult  are  produced  in  the  meshes  of  reticular 
tissue  outside  of  the  blood  vessels;  the  l}Tnphocytes  chiefly  in  the  lymph 
glands,  and  the  granular  leucocytes  chiefly  in  the  red  bone  marrow  where 
the  red  corpuscles  also  develop.  The  lymphocytes  appear  in  the  circu- 
lation before  the  granular  leucocytes.  An  investigator  (Engel)  of  the 
blood  in  pig  embryos  found  that  well  defined  leucocytes  similar  to  lympho- 
cytes appeared  first  in  pigs  of  8  cms.  Another  investigator  (Sabin)  has 
recorded  that  in  the  lymph  glands  of  an  8  cm.  pig  the  lymphocytes  are  first 
recognizable.  From  these  independent  studies  it  seems  that  lymphocytes 
appear  in  the  lymph  glands  and  in  the  blood  at  about  the  same  time. 
"  In  the  guinea  pig  there  seems  to  be  a  connection  between  the  time  of  the 
appearance  of  the  polymorphonuclear  leucocytes  in  the  marrow  and  in 
the  blood"  (Jolly  and  Acuna).  The  granular  leucocytes  appear  in  the 
blood  and  in  the  marrow  at  first  as  cells  with  round  nuclei.     Such  cells 


^TECITE    CORPUSCLES.  I49 

in  the  adult  are  found  normally  only  in  the  marrow  and  are  called  myelo- 
cytes. They  enter  the  blood  when  their  protoplasm  is  full  of  the  granules 
which  develop  gradually,  and  when  their  nuclei  are  polymorphous.  Only 
in  disease  are  myelocytes  and  er}^throblasts  found  in  the  blood  of  adults 
but  they  circulate  normally  in  the  blood  of  young  embryos.  The  important 
question,  whether  the  leucocytes  arise  directly  from  the  mesench}Tiial 
tissues  of  hmph  gland  and  bone  marrow,  or  from  cells  which  have  emi- 
grated into  them  from  the  blood  vessels,  has  not  been  determined. 

The  large  mononuclear  cells  with  round  nuclei  are  thought  by  some 
to  be  cells  from  which  both  lymphocytes  and  granular  forms  arise.  The 
granules  may  be  ^  secretory  products.  Eosinophihc  granules  were  once 
thought  to  be  transformations  of  the  neutrophihc,  occurring  in  old  cor- 
puscles. Lately  they  have  been  regarded  as  the  ingested  fragments  of 
red  corpuscles,  but  the  fact  that  they  rarely,  if  ever,  are  mixed  with 
neutrophihc  granules  is  against  this  view.  The  form  of  granule  seems 
to  be  determined  by  unkno\\Ti  factors  early  in  the  differentiation  of  the 
leucocytes,  and  to  be  fixed  for  a  given  cell  after  the  first  granules  have 
appeared. 

In  connection  with  the  terms  apphed  to  leucocytes  it  should  be  noted 
that  those  with  basophile  granules  are  not  caUed  basophiles  as  would  be 
consistent,  but  mast  cells.  The  non-granular  l}miphocytes  and  large  mono- 
nuclear cells  are,  however,  sometimes  called  basophiles  because  their 
protoplasm  takes  a  pale  basic  stain.  This  is  undesirable.  Mast  cells 
were  originally  caUed  plasma  cells,  a  term  now  apphed  to  oval  cells  derived 
from  lymphocytes  by  an  increase  in  their  protoplasm  (Fig.  49,  p.  47). 
They  have  eccentric  nuclei,  and  their  non-granular  protoplasm  stains  deeply 
with  basic  dyes.  Plasma  cells  occur  in  connective  tissue,  but  probably 
not  in  the  blood;  they  are  of  pathological  importance. 

The  varieties  of  leucocytes  may  be  re\dewed  as  follows: 

Lymphocytes,  22  to  25  %  of  the  leucocytes,  are  small  (about  the  szie 
of  a  red  corpuscle)  or  large  (perhaps  twice  the  diameter  of  a  red  corpuscle), 
non-granular,  with  round  checkered  nuclei. 

Large  mononuclear  leucocytes,  i  to  3  %,  may  be  two  or  three  times  the 
diameter  of  red  corpuscles.  They  are  non-granular,  or  mth  few  granules, 
and  have  pale  vesicular  nuclei,  round  or  crescentic. 

Polymorphonuclear  leucoc}1;es,  larger  than  red  corpuscles,  are  gran- 
ular, with  nuclei  variously  constricted  or  bent.     They  include, — 

Mast  cells,  0.5  %,  mth  very  coarse  basophihc  granules  obscuring 

the  nucleus. 
Eosinophiles,  2  to  4  %,  with  coarse  eosinophihc  granules. 
Neutrophiles,  70  to  72  %,  with  fine  neutrophihc  granules. 


I^O 


HISTOLOGY. 


Fig.  172.  —  Blood 
Plates  beside 
A  Red  Cor- 
puscle. 


Blood_;^lates^  or  platelets  are  round  or  irregular  protoplasmic  structures, 
2  to  4  a  in  diameter.  From  245,000  to  778,000  have  been  estimated  to  occur 
in  a  cubic  millimeter  of  human  blood.  They  are  readily  reduced  to  gran- 
ular debris  in  ordinary  sections  but  when  well  preserved  and  stained  with 
Wright's  blood  stain  it  appears  that  they  have  dark  granular  centers  and 
clear  peripheral  zones  (Fig.  172).  They  have  formerly 
..ssff;  been  interpreted  as  small  nucleated  cells,  and  as  fragments 
of  leucocytes.  Dr.  J.  H.  Wright  has  recently  shown  that 
they  are  fragments  of  elongated  pseudopodia  of  the  giant 
cells  in  the  bone  marrow.  Their  peripheral  zone  is  ecto- 
plasm and  their  inner  granular  part  is  endoplasm.  Con- 
sequently they  are  non-nucleated.  The  giant  cells  are  not 
always  producing  blood  plates.  Only  certain  of  them 
show  the  pseudopodia,  which  have  been  observed  extending 
into  the  blood  vessels.  In  the  blood  the  plates  exist  for  some  time,  as  they 
are  found  in  clots  several  days  old.  The  function  of  the  plates  is  unknown. 
In  drawn  blood  they  rapidly  adhere  to  one  another  forming  masses,  but  not 
rouleaux.  Sometimes  they  present  irregular  projections  and  so  have  been 
described  as  amoeboid. 
In  the  clotting  of  blood 
the  plasma  separates 
into  a  sohd  part,  the 
'fihrin  and  a  thin  fluid, 
the  serum.  The  blood 
clot  or  thrombus  con- 
sists of  fibrin  with  the 
entangled  corpuscles,  a 
mass  which  contracts 
after  it  forms,  squeezing 
out  the  serum.  The 
fibrin  is  deposited  (pre- 
cipitated ?)  in  slender 
threads  which  radiate 
from  the  blood  plates 
and  form  nets  shown  in 
Fig.  173.  Therefore 
the  plates  have  been 
considered  active  agents 

in  the  clotting  of  blood  and  have  been  called  thromboc}'tes.  In  the  blood 
of  amphibia,  spindle  shaped  nucleated  cells  smaller  than  their  red  corpus- 
cles possess  adhesive  properties  and  are  also  named  thrombocytes.     Since 


Fig.  173.  —  Red  Corpuscles  forming  Rouleaux.  Fibrin  in 
F'lLAMENTS  Radiating  from  Blood  Plates.  (From  Da 
Costa's  Clinical  Hematology.) 


PLASilA.  151 

the  plates  have  been  sho\\Ti  to  be  fragments  of  giant  cells  they  can  scarcely 
be  homologous  with  the  amphibian  thrombocytes. 

Plasma  is  the  fluid  intercellular  substance  of  the  blood.  It  contains 
various  granules  some  of  which  are  small  fat  drops  received  from  the 
thoracic  duct.  Others  occurring  in  variable  quantity  are  refractive  parti- 
cles, not  fatty,  either  round  or  elongated;  they  are  knov,Ti  as  haemato- 
konia  (or  haemokoniaj.  In  ordinary  sections  the  plasma  appears  as  a 
granular  coagulum. 

LvilPH. 

The  contents  of  the  lymphatic  vessels  is  called  lymph.  It  is  a  fluid 
which  may  contain  the  various  cellular  elements  of  blood  in  small  numbers. 
Red  corpuscles  and  polymorphonuclear  leucocytes  are  occasional.  Lym- 
phocytes are  the  most  abundant  cells,  and  some  of  them  have  considerable 
protoplasm  and  are  phagocytic.  The  lymph  fluid  is  not  identical  with 
plasma  or  with  tissue  fluids,  yet  all  three  are  similar.  Nutrient  material 
from  the  plasma  traverses  the  tissue  fluids  to  the  epitheHal  ceUs,  certain 
products  of  which  pass  back  into  the  tissue  fluids.  They  may  be  taken 
up  by  the  blood  or  by  the  hnnph,  first  passing  through  the  endothehal  cells 
of  the  vessels.  From  the  intestine  much  of  the  absorbed  fat  is  transferred 
across  the  tissue  spaces  to  the  hmiphatic  vessels  in  which  it  forms  a  milky 
emulsion  knoAvn  as  chyle.  (The  small  lymphatic  vessels  containing  it  have 
been  known  as  lacteals.)  This  example  shows  that  hnnph  may  exist  in 
more  than  one  form.  In  the  subclavian  veins  it  mingles  with  the  blood 
plasma. 

In  ordinary  sections  hmiph  appears  as  a  fine  coagulum,  containing  a 
few  lymphocytes,  and  occasionally  other  corpuscles. 


III.  SPECIAL  HISTOLOGY. 


BLOOD  FORMING  AND  BLOOD  DESTROYING  ORGANS. 


iy— my 


Bone  Marrow. 

Bone  marrow  is  the  soft  tissue  found  within  the  central  cavities  of 
bones.  Its  source  in  the  embryo  is  the  vascular  mesenchyma  invading  a 
cartilage  which  is  being  replaced  by  bone.  Early  in  its  development  it 
contains  osteoblasts  and  osteoclasts,  and  these  cells  may  be  found  in  adult 
marrow  where  it  is  in  contact  with  the  bone.  The  greater  part  of  the 
mesenchyma  becomes  reticular  tissue  with  fat  cells  intermingled.  The 
meshes  of  the  reticular  tissue  are  occupied  by  an  extraordinary  variety  of 
cells,  most  of  which  are  called  myelocytes  (marrow  cells).     In  ordinary 

sections  the  marrow  appears  as  a  com- 
pact tissue  of  small  cells  riddled  with 
large  round  holes.  Under  high  magnifi- 
cation the  holes  are  seen  to  be  fat  cells 
the  nuclei  of  which  are  here  and  there 
included  in  the  section  (Fig.  174).  The 
reticular  framework  of  the  marrow  con- 
sists of  flattened  cells  generally  seen  cut 
across  ;theirnucleithenappear  slender  and 
elongated.  The  abundant  meshwork  of 
fibrils  associated  with  these  cells  is  not  ap- 
parent in  ordinary  sections.  In  the  meshes 
are  found  giant  cells;  premyelocytes;  my- 
elocytes which  are  neutrophilic,  basophilic 
or  eosinophilic;  erythrocytes;  lymphocytes; 
and  mature  corpuscles  both  red  and  white. 
The  giant  cells  of  the  marrow  have  a  single  polymorphous  nucleus. 
They  have  been  named  therefore  'megakaryocytes,'  in  distinction  from 
the  multinucleate  osteoclasts  or  'polykaryocytes.'  The  nucleus  is  so 
large  that  it  may  be  cut  into  several  sHces,  and  by  combining  these  it  has 
been  found  that  the  entire  nucleus  is  a  hollow  sphere  with  perforated  walls. 
The  nuclei,  however,  are  very  irregular  and  some  may  be  of  other  forms. 
With  Wright's  stain  the  protoplasm  clearly  shows  an  outer  hyahne  exo- 
plasm  and  an  inner  granular  endoplasm.     It  has  been  said  that  the  latter 

[152 


Fig.  174. — Human  Bone  Marrow. 
e.,  Eosinophilic  myelocyte;  e-b.,  erythro- 
blast ;  e-C,  erythrocyte  ;  f.  c.,  part  of  the 
protoplasmic  rim  of  a  fat  cell;  g.  c, 
giant  cell ;  my.,  neutrophylic  myelocyte  ; 
n-b.,  normoblast ;  pm.,  premyelocyte  ;  r., 
reticular  tissue  cell. 


BONE    MARROW.  153 

is  divisible  into  two  concentric  zones,  which  differ  from  the  protoplasm 
within  the  nuclear  sphere.  In  ordinary  preparations  these  details  are 
not  evident.  A  large  number  of  centrosome  granules  (over  one  hundred) 
has  been  found,  and  pluripolar  mitoses  have  been  observed.  A  phago- 
cytic function  has  been  ascribed  to  these  giant  cells,  but  it  has  also  been 
denied.  Their  origin  is  unknown,  but  is  said  to  be  from  the  leucocyte 
series  of  cells.  The  important  function  of  producing  blood  plates  has  but 
recently  been  estabhshed  (see  page  150). 

Premyelocytes  are  cells  with  large  round  vesicular  nuclei  containing 
one  or  two  coarse  chromatin  masses,  and  surrounded  by  basic  protoplasm 
free  from  specific  granules.  It  is  possible  that  these  cells  are  parents  of 
myelocytes. 

Myelocytes  are  cells  larger  than  polymorphonuclear  leucocytes,  hav- 
ing round  or  crescentic  nuclei  and  protoplasm  containing  a  varying  quan- 
tity of  specific  granules,  either  neutrophihc,  basophihc,  or  eosinophihc. 
The  young  cells  have  round  nuclei  and  few  granules.  The  oldest  become 
the  granular  leucocytes  ready  to  enter  the  blood  vessels.  Several  genera- 
tions derived  by  mitosis  intervene  between  the  young  myelocytes  and  the 
mature  leucocytes.  Most  of  the  myelocytes  are  finely  granular  and  neu- 
trophihc. Some  are  coarsely  granular  and  eosinophihc;  others  contain 
the  basophihc  mast  cell  granules,  but  these  are  not  well  preserved  in  ordi- 
nary specimens.  In  certain  diseases  myelocytes  enter  the  circulating 
blood,  and  they  appear  in  smears  as  sho\^^l  in  Fig.  171,  p.  147. 

Erythrocytes  are  generally  found  in  clusters,  some  being  young  with 
vesicular  nuclei,  others  being  normoblasts  with  dense  irregular  nuclei  such 
as  have  already  been  described.  Rarely  a  nucleus  may  be  found  which 
apparently  is  partly  extruded.  Cup  shaped  corpuscles  are  seen  in  the 
tissue  meshes. 

Lymphocytes  are  not  a  conspicuous  element  of  the  marrow,  yet  they 
are  present  and  sometimes  in  disease  become  abundant. 

The  relations  of  the  blood  vessels  to  the  reticular  tissue  are  of  great 
interest.  It  has  been  thought  that  the  endothehum  blends  with  the  retic- 
ulum so  that  no  sharp  distinction  can  be  made  between  the  two.  It  seems 
more  probable  that  the  endothelium  is  merely  more  permeable  than  usual, 
by  a  freer  separation  of  its  cells.  The  same  problem  is  presented  by  the 
blood  vessels  and  reticular  tissue  of  the  lymph  glands  and  spleen. 

The  functions  of  the  marrow  are  the  production  and  dissolution  of  the 
bone,  the  storing  of  fat,  the  formation  of  granular  leucocytes  (neutrophiles, 
eosinophiles,  and  mast  cells),  of  red  corpuscles,  and  to  a  less  extent  of 
lymphocytes;  to  these  some  would  add  the  destruction  of  red  corpuscles  as 
indicated  by  ingested  fragments  and  intercellular  granules. 


154  HISTOLOGY. 

Such  marrow  as  has  just  been  described  is  called  7'ed  marrow.  It 
occurs  in  the  bones  of  embryos  and  persists  in  the  flat  bones  of  the  adult, — 
those  of  the  skull,  the  bodies  of  the  vertebrae,  the  ribs  and  sternum, 
the  epiphyses,  and  the  heads  of  the  humerus  and  femur.  The  shafts  of  the 
long  bones  contain  yellow  marroiv  which  resembles  ordinary  fat  tissue. 
Between  the  fat  cells  an  occasional  plasma  cell  or  myelocyte  may  occur. 
Yellow  marrow  is  formed  from  red  by  the  development  of  true  fat  cells 
and  not  by  fatty  degeneration  of  myelocytes.  In  disease  it  may  resume 
its  blood  forming  function  and  become  red.  In  starvation  it  becomes 
mucoid  like  other  fat  tissue. 


Lymph  nodules  and  Lymph  glands. 

Lymph  glands,  haemolymph  glands,  and  the  spleen  have  a  similar 
origin  in  the  embryo.  They  are  at  iirst  small  dense  areas  of  mesenchyma 
developing  near  blood  and  lymphatic  vessels.  The  blood  vessels  extend 
into  these  areas  producing  a  notch  on  one  side  of  the  mass,  known  as  the 
hilus.  Here  in  the  adult  the  arteries  enter  and  veins  leave.  After  the 
invasion  of  the  blood  vessels  the  dense  tissue  is  tranformed  into  reticular 
tissue  containing  lymphocytes.  The  lymphocytes  occur  especially  in  that 
part  which  surrounds  the  arteries.  The  veins  tend  to  be  at  the  periphery 
of  the  compact  lymphoid  tissue  surrounding  the  arteries  and  to  be  asso- 
ciated with  a  portion  of  the  reticulum  which  is  comparatively  free  from 
lymphocytes.  Lymphatic  vessels  spread  over  the  surface  and  into  the 
substance  of  the  lymph  glands,  but  they  are  absent  from  haemolymph 
glands  and  from  the  spleen. 

Lymph  glands  (also  called  lymph  nodes)  in  early  stages  of  develop- 
ment are  shown  in  Fig.  175,  the  left  half  of  which  represents  a  younger 
stage  than  the  right.  The  left  portion  shows  a  mass  of  reticular  tissue 
and  lymphocytes  penetrated  by  an  artery  and  a  vein  which  join  through 
capillaries.  It  is  surrounded  by  a  network  of  lymphatic  vessels  some  of 
which  are  afferent  and  others,  toward  the  hilus,  are  efferent.  Such 
structures  occurring  in  the  adult  are  called  solitary  nodules  [follicles].  They 
are  abundant  in  the  walls  of  the  intestine  and  respiratory  tubes.  Each 
is  an  area  of  lymphocyte  production  characterized  by  crowded  nuclei 
which  stain  deeply  with  haematoxyhn.  Under  low  magnification  the 
nodule  appears  as  a  mass  of  dark  granules  (Fig.  244,  p.  216)  in  the  center 
of  which  a  hghter  area  is  sometimes  seen,  the  germinative  center.  Here 
the  cells  are  larger,  resembling  the  large  mononuclear  leucocytes  of  the 
blood,  and  are  frequently  found  in  mitosis.  They  are  thought  to  give 
rise  to  lymphocytes.     The  reticular  tissue,  which  is  concealed  by  the  cells 


LYMPH   GLANDS. 


155 


in  its  meshes,  forms  a  coarser  net  in  the  germinative  centers  than  in  the 
peripheral  part  of  the  nodule.  Blood  vessels  within  the  nodule  are  incon- 
spicuous and  the  surrounding  lymphatic  vessels  are    sometimes   absent. 

AfEerent    lymphatic    vessels. 

i/ 


Lymphoid  tissue 


/  Blood  vessels.  J^  \^ 

Lymphatic  vessel.  '  Lymphatic  vessel. 

Fig.  175. 


Peripheral 
lymph  sinus. 


Lymph  sinus. 


Peripheral   sinus. 


Lymph  sinus. 


Afferent  lymphatic  vessels. 


Capsule,  y 


Medullary  cord. 


Trabecula. 


Efferent  lymphatic  vessels. 

Fig.  176. 
Diagrams  Representing  Four  Stages  in  the  Development  of  Lymph  Glands. 

Certain  of  the  solitary  nodules  are  merely  transient  local  accumulations 
of  lymphocytes  which  are  diffusely  distributed  in  the  layer  of  reticular  tissue 
found  beneath  the  intestinal  epithelium. 


156  HISTOLOGY. 

In  the  small  intestine  and  in  the  vermiform  process,  lymphatic  nodules 
occur  side  by  side,  so  as  to  form  macroscopic  areas  visible  on  the  inner 
surface  of  the  intestine.  They  are  broadly  elliptical,  and  usually  from 
I  to  5  cms.  long  though  occasionally  much  longer.  From  two  to  forty  or 
more  nodules  may  enter  into  the  formation  of  one  of  these  aggregate  nod- 
ules [Peyer's  patches]  and  they  may  remain  distinct  though  adjacent,  as 
in  Fig.  241,  p.  213,  or  they  may  be  confluent.  In  the  latter  case  they  may 
be  recognized  by  their  germinative  centers.  Their  structure  is  that  of 
the  sohtary  nodules. 

The  lymph  glands  are  round  or  bean  shaped  structures,  varying  in 
length  from  a  few  millimeters  to  a  few  centimeters.  They  occur  along  the 
courses  of  the  lymphatic  vessels,  as  is  shown  in  text  books  of  anatomy. 
In  producing  a  lymph  gland,  as  seen  on  the  right  of  Fig.  175,  a  connect- 
ive tissue  capsule  forms  around  the  lymphoid  tissue,  into  which  it  later 
sends  trabeculae  and  plate-like  prolongations.  These  may  unite  with 
similar  trabeculae  from  the  region  of  the  hilus,  as  on  the  right  of  Fig.  176, 
thus  making  columns  of  connective  tissue  extending  from  one  side  of  the 
gland  to  the  other.  Such  a  complete  trabecular  system  is  found  only  in 
the  larger  lymph  glands.  The  capsule  consists  of  connective  tissue  with 
elastic  elements  which  increase  with  age.  It  contains  also  scattered 
smooth  muscle  fibers;  the  trabeculae  are  of  similar  structure. 

Beneath  the  capsule  and  surrounding  the  trabeculae,  there  is  a  reticular 
meshwork  comparatively  free  from  lymphocytes.  This  is  called  the  lymph 
sinus.  It  is  in  free  communication  with  the  afferent  and  efferent  lym- 
phatic vessels,  and  is  also  continuous  with  the  reticulum  of  the  dense 
lymphoid  tissue.  Its  embryological  relation  to  the  lymphatic  vessels  has 
not  been  satisfactorily  determined.  Some  consider  that  it  is  a  network  of 
endotheUal  tubes  closely  investing  slender  strands  of  reticular  tissue;  others 
beheve  that  the  endothelial  tubes  are  penetrated  by  the  reticular  tissue; 
and  still  others  that  the  endothehum  blends  inseparably  with  the  reticulum, 
into  which  the  lymphatic  vessels  therefore  open  freely.  It  seems  justifiable 
to  maintain  that  endothelium  and  reticular  tissue  are  distinct,  though  in 
close  relation.  All  of  the  functions  and  appearances  of  the  sinus  can  be 
explained  if  the  endothehal  lymphatic  vessels  are  regarded  as  freely  per- 
meable in  the  gland,  by  separation  of  their  cells  from  one  another.  Fig. 
178  shows  the  trabeculae  highly  magnified;  between  them  and  the  dense 
lymphoid  tissue  are  the  lymph  sinuses. 

Several  organs  can  be  divided  into  an  outer  and  an  inner  portion, 
called  cortex  (meaning  bark)  and  medulla  (pith)  respectively.  The  lymph 
gland  is  one  of  these.  Its  cortical  part,  shown  in  Figs.  176  and  179,  con- 
sists of  large  lymphoid  masses  resembling  nodules  and  containing  germi- 


LYMPH    GLANDS. 


157 


native  centers.  These  are  sometimes  called  secondary  nodules.  The 
medullary  portion  includes  cord-like  prolongations  of  the  nodules,  called 
medullary  cords.  The  secondary  nodules  often  are  incompletely  separated 
from  one  another  and  the  cords  join  to  form  a  network.  Both  the  nodules 
and  the  cords  are  enveloped  by  the  lymph  sinuses,  and  the  trabeculae  if 
present  are  in  the  midst  of  the  sinuses  (Figs.  177  and  178).  The  nodules 
and  cords  are  both  composed  of  lymphocytes  in  a  close-meshed  reticular 
tissue. 

The  blood  vessels  of  the  lymph  gland  in  part  enter  from  various 
points  in  the  capsule  and  run  in  the  trabeculae,  but  the  chief  vessels  enter 
at  the  hilus.     The  arterv  divides  into  several  branches  which  remain  in 


Fig.  177.  Fig.  178. 

From  Vertical  Sections  through  the  Medulla  of  a  Lymph  Gland  of  an  Ox. 
Fig.  177,  X  5O1  shows  the  medullary  cords  and  trabeculae  cut  lengthwise  in  its  upper  part,  and  cut  across 
in  its  lower  part.     Both  the  cords  and  the  trabeculae  form  continuous  networks.     Fig.  178,  X  240, 
shows  the  fine  reticular  tissue  of  the  lymph  sinus,  containing  a  few  leucocytes. 

the  trabeculae  for  only  a  short  distance,  and  then  cross  the  lymph  sinuses 
to  the  medullary  cords.  They  extend  through  the  axes  of  the  cords  into 
the  nodules,  giving  off  small  branches  which  form  capillary  networks  and 
unite  in  veins  found  at  the  periphery  of  the  nodules  and  cords.  The  veins 
soon  cross  the  sinuses  and  enter  the  trabeculae  in  which  they  travel  toward 
the  hilus.  A  central  artery,  surrounded  by  lymphoid  tissue  together  with 
peripheral  veins,  is  found  not  only  in  lymph  glands  but  also  in  the  spleen. 
The  lymphatic  vessels  penetrate  the  capsule  at  several  points  and  become 
involved  in  the  lymph  sinuses.  Through  these,  partly  in  endothehal  tubes, 
and  partly  in  tissue  spaces,  the  lymph  flows  toward  the  hilus  which  it 


158  HISTOLOGY. 


leaves  in  the  efferent  vessels,  fewer  in  number  than  the  afferent.     Lympho- 
cytes are  added  to  the  lymph  as  it  passes  across  the  gland. 

Nerves  to  the  lymph  glands  are  not  abundant.  They  consist  of  medul- 
lated  and  non-medullated  fibers  which  form  plexuses  about  the  blood 
vessels,  and  supply  the  muscle  cells  in  the  capsule  and  trabeculae.  They 
have  not  been  found  in  the  nodules  and  cords. 


X 


Capsule. 
Trabeculae. 


Hilus. 


Cortical 
substance. 


Medullary 
substance. 


Germinal  -  "  Fat. 

center  of  a      - 
secondary 
nodule. 


Fig.  179.— Longitldinal  Section  of  a  Hi.man  Cervical  Lymph  Gland.    X  12. 

The  function  of  the  lymph  glands  is  not  only  to  produce  lymphocytes 
which  enter  the  lymphatic  vessels,  but  also  to  "filter  the  lymph."  If  cer- 
tain poisonous  substances,  inert  particles,  or  bacteria  are  brought  to  the 
gland  in  the  lymph,  they  may  be  removed  by  phagocytic  endothehal  or 
reticular  cells.  The  gland  at  the  same  time  may  become  enlarged  by  con- 
egstion,  and  by  multiplication  of  its  cells. 


haemolymph  glands.  1 59 

Haemolymph  glands. 

Haemolymph  glands  resemble  lymph  glands  in  form  and  also  in  size, 
ranging  from  that  of  a  "pinhead  to  an  almond."  They  occur  espe- 
cially in  the  retro-peritoneal  tissue  near  the  origin  of  the  superior  mesenteric 
and  renal  arteries,  but  also  in  the  thorax  and  neck.  They  are  darker  than 
lymph  glands,  and  on  section  yield  blood  in  place  of  lymph.  No  lymphatic 
vessels  are  associated  with  typical  haemolymph  glands,  and  instead  of  a 
lymph  sinus  they  possess  a  similar  structure  filled  with  blood,  the  hlood 
sinus.  The  lymphoid  tissue  with  its  blood  supply,  together  with  the  cap- 
sule and  trabeculae,  are  hke  the  corresponding  structures  in  lymph  glands. 
The  capillary  blood  vessels,  however,  are  readily  permeable  so  that  their 
contents,  both  plasma  and  corpuscles,  escape,  into  the  blood  sinus.  The 
haemolymph  gland  is  therefore  a  blood  filter.  Many  blood  corpuscles 
fragment  and  are  removed  from  the  circulation  by  phagocytic  cells  which 
in  consequence  become  pigmented.  Eosinophilic  cells  which  have  been 
found  in  haemolymph  glands  have  been  explained  as  due  to  the  ingestion 
of  haemoglobin  products.  Haemolymph  glands  have  as  a  second  function 
that  of  producing  lymphocytes  which  may  enter  the  blood  vessels. 

After  accidents  accompanied  by  extravasations  of  blood,  the  lymph 
sinuses  of  lymph  glands  may  be  filled  with  red  corpuscles  conveyed  to  them 
by  afferent  lymphatics.  Such  glands  should  not  be  confounded  with 
haemolymph  glands  which  have  no  lymphatic  vessels.  It  has  been  said, 
however,  that  intermediate  forms  between  the  two  sorts  of  glands  occur, 
meaning  that  some  normal  lymphatic  glands  contain  blood  in  their  sinuses 
derived  from  their  own  blood  vessels.  The  embryology  of  haemolymph 
glands  is  unknown  but  it  is  not  supposed  that  they  are  lymph  glands  which 
in  the  course  of  development  have  lost  their  lymphatic  vessels.  They  are 
regarded  rather  as  structures  which  are  distinct  from  the  outset,  and  which 
are  closely  related  to  the  spleen. 

Spleen. 
The  spleen,  being  five  or  six  inches  long  and  four  inches  wide,  is  much 
the  largest  organ  of  the  lymph  gland  series.  It  is  the  first  of  them  to  de- 
velop, appearing  in  rabbits  of  14  days  (10  mm.)  as  a  condensation  of  the 
mesenchyma  in  the  dorsal  mesentery  of  the  stomach.  At  this  stage  the 
only  lymphatic  vessels  in  the  embryo  are  those  near  the  jugular  vein. 
Lymph  glands  are  not  indicated  until  six  days  later.  The  blood  vessels 
enter  the  spleen  at  its  hilus  and  branch  freely.  It  is  unknown  whether  or 
not  the  artery  ever  connects  with  the  vein.  Surrounding  the  arterial 
branches  there  is  a  zone  of  lymphoid  tissue  which  is  so  highly  developed 


i6o 


HISTOLOGY. 


in  reptilian  spleens  that  they  resemble  closely  mammalian  haemolymph 
glands.  In  the  guinea  pig  the  lymphoid  sheath  of  the  arteries  is  continuous, 
though  narrow;  in  man  it  is  so  interrupted  as  to  form  a  succession  of  spindle- 
shaped  or  spherical  masses  called  splenic  nodules  [Malpighian  corpuscles]. 
An  arterial  branch  passes  through  each  nodule.  Thus,  as  compared  with 
the  haemolymph  gland,  the  spleen  is  deficient  in  lymphoid  tissue  (Fig.  i8o). 
The  bulk  of  the  spleen  is  composed  of  splenic  pulp,  which  corresponds 
with  the  blood  sinus  of  the  haemolymph  glands.  Its  framework  of  retic- 
ular tissue  is  continuous  with  that  of  the  nodules,  and  it  contains  blood  cor- 
puscles of  all  sorts,  special  phagocytic  cells  known  as  splenic  cells,  and  the 
terminal  branches  of  both  arteries  and  veins.  There  are  no  lymphatic 
vessels  within  the  spleen.  The  capsule  and  trabecular  framework  are 
highly  developed  as  in  the  largest  lymph  glands.     The  following  features 


spn 


art  tr— 

A  B 

Fig.  iSo.— Diagram  ok  a  Haemolymph  Gland,  A,  and  op-  a  Part  of  the  Spleen,  B. 
The  arteries  are  shown  as  slender  lines  (art.)  and  the  veins  as  heavy  ones  (v.);  c,  capsule;  b.  S.,  hlood 
sinus,  corresponding  with  the  splenic  pulp,  p.;  s.  n.,  secondary  nodule;   sp.  n.,  splenic  nodule;  tr., 
trabecula. 


of  the  spleen  may  be  described  in  turn;  the  blood  vessels,  the  pulp,  the 
nodules,  the  capsule  and  trabeculae,  and  finally  the  nerves. 

As  shown  in  the  diagram,  Fig.  i8i,  the  splenic  artery  enters  at  the 
hilus  and,  accompanied  by  veins,  its  branches  are  found  in  the  largest  tra- 
beculae. When  about  0.2  mm.  in  diameter  the  arteries  leave  the  trabec- 
ulae in  which  the  veins  continue  further.  The  arteries,  however,  are  still 
surrounded  by  a  considerable  connective  tissue  layer,  the  outer  portion  of 
which  becomes  reticular  and  filled  with  the  lymphocytes  of  the  nodules. 
The  nodules  occur  near  where  the  artery  branches.  Small  arterial  twigs 
ramify  in  the  nodules,  in  the  periphery  of  which  they  anastomose  before 
passing  on  to  the  pulp.  When  the  main  stems  are  about  15  //  in  diameter 
they  lose  their  surrounding  lymphoid  layer  and  pass  into  the  pulp  where 
they  form  brush-hke  groups  of  branches  {penicilli).     These  branches  do 


SPLEEN. 


i6i 


not  anastomose.     For  a  short  distance  before  their  termination  the  walls 
of  these  branches  possess  elhpsoid  thickenings  due  to  a  longitudinal  ar- 


Terminal  vein. 


Sheathed  arterj-        Pu'P  artery-. 


Pulp  vein. 


Trabecula. 


Penicillus. 


\  Spleen 
'      lobule. 


Capsule. 
5lood  Vessels  of  the  Human  Spleen. 


Reticulum.  Splenic  nodule. 


Fig.  iSi. — Dl\gram  of  the 
At  X  is  shown  the  direct  connection  of  terminal  arteries  with  terminal  veins   (the  existence  of  such  a 
connection  has  been  questioned  i.     At  XX  and  XXX  are  the  free  endings  of  the  terminal  veins  in  the 
pulp  and  near  the  nodules  respectively. 

rangement  of  closely  apphed  reticular  fibers.     These  '  sheathed  arteries' 
are  6-8  y.  in  diameter,  and  have  been  supposed  to   regulate  the  amount 
of    blood  which  enters  the 
distal  portion  of  tiie  artery.  A      ■   ^ 

Some  authorities  state  that 
this  distal  part  connects 
with  the  terminal  veins, 
meeting  them  at  an  acute 
angle.  According  to  others 
such  connections  are  infre- 
quent, and  still  others 
believe  that  the  arteries 
empty  only  into  the  reticu- 
lar tissue.  Numerous  care- 
ful injections  have  shown 
the  readiness  with  which  the  arterial  blood  mingles  with  the  pulp  cells. 


Fig.  1S2.— Cross  Section  ^A"!  and  Surface  View  fB)  of 
Terminal  \'eins  from  the  Human  Spleen. 
e.,  Rod  shaped  endothelial  cells,  with  projecting  nuclei,  n:  r., 
encircling  reticular  tissue;   I.,  leucocytes  passing  between 
the  endothelial  cells.      (After  Weidenreich.) 


l62 


HISTOLOGY. 


The  terminal  veins  begin  as  dilated  structures  (sometimes  unfortu- 
nately called  '  splenic  sinuses,'  or '  ampullae,'  the  latter  term  being  applied 
also  to  the  terminal  arteries).  Their  endothelial  cells  are  so  long  and  slen- 
der as  to  suggest  smooth  muscle  fibers,  and  like  certain  other  endothehal 
cells  they  are  contractile.  Their  edges  are  not  closely  approximated,  so 
that  corpuscles  may  pass  between  them  freely  as  shown  in  Fig.  182.  Around 
them  are  encirchng  reticular  tissue  fibers,  and  a  continuous  basement  mem- 
brane-like structure  has  been  described  stretching  across  the  intervals  be- 
tween the  endothehal  cells.  The  existence  of  such  a  membrane  has  re- 
cently been  denied.    A  peculiar  feature  of  the  endothelial  cells  is  their 


Pulp 


Vg       Capsule. 

I 

■^•"^^    Trabeculae. 


Spindle  shaped 
nodule.  ^ 


Sheathed  arterv. 


Central  arteries 
in  splenic  nodules. 


Fig.  183. — Part  of  a  StxTiox  of  the  Spleen  from  an  Adllt  Ma.v. 


projection  into  the  lumen  of  the  vessel,  their  nuclei  being  at  the  summits 
of  these  elevations  as  sho^^-n  in  Fig.  182.  Several  terminal  veins  unite  to 
form  a  pulp  vein  which  enters  a  trabecula  in  which  it  passes  toward  the 
hilus.     The  trabecular  veins  join  to  form  the  splenic  vein. 

The  splenic  pulp  consists  of  a  reticular  tissue  framework  such  as  has 
been  described  on  p.  39.  It  supports  the  terminal  arteries  and  the  term- 
inal and  pulp  veins,  and  in  its  meshes  are  the  white  and  red  corpuscles 
passing  between  them. 

The  pulp  appears  as  a  diffuse  mass  of  cells  infiltrated  with  red  cor- 
puscles, and  since  the  vessels  within  it  are  thin  walled  and  hard  to  follow, 
likewise  containing  corpuscles,  it  is  often  impossible  in  ordinary  sections  to 


SPLEEN.  163 

determine  which  cells  are  inside  and  which  are  outside  of  the  vessels  (Fig. 
183) .  The  nodules  are  not  shai-ply  separated  from  the  pulp,  so  that  lympho- 
cytes are  abundant  in  their  ci'^inity.  These  lymphocytes  enter  the  term- 
inal veins  and  thus  are  removed  from  the  spleen.  In  the  splenic  vein  the 
proportion  of  lymphocytes  to  red  corpuscles  is  said  to  be  seventy  times  as 
great  as  in  the  splenic  artery.  One  for  every  four  red  corpuscles  has  been 
reported  by  two  investigators,  but  later  estimates  are  lower.  It  seems  evi- 
dent that  lymphocyte  production  is  an  important  function  of  the  spleen. 
Another  is  the  filtration  of  the  blood  passing  through  the  pulp.  As  in 
haemolymph  glands  granular  debris  is  found,  and  there  are  phagocytic, 
pigmented,  and  eosinophilic  cells.  The  phagocytes  are  cells  vrith  large 
round  nuclei  and  considerable  protoplasm.  They  vary  in  size,  but  the 
small  forms  are  most  numerous;  these  are  called  splenic  cells.  Some  are 
described  as  multinucleate.  Erythroblasts  are  not  found  in  the  normal 
adult  human  spleen;  in  certain  blood  diseases,  however,  they  occur  in  it 
and  are  normal  in  some  adult  mammals,  as  in  the  skunk.  They  are  abun- 
dant in  the  spleens  of  human  embryos.  Giant  cells  are  numerous  in  the 
spleens  of  young  animals  but  are  seldom  found  in  the  human  adult.  They 
are  described  as  megakarj^ocytes.  The  formation  of  granular  leucocytes, 
which  has  been  asserted,  presumably  does  not  occur. 

The  splenic  nodules  are  quite  like  the  secondary  nodules  of  lymph 
glands.  They  consist  of  a  reticular  tissue  framework  continuous  with  that 
of  the  pulp,  but  having  coarser  meshes.  Fine  elastic  fibers  are  associated 
with  it.  It  contains  lymphocytes,  and  near  the  central  arteries  germinative 
centers  are  sometimes  distinct.  The  nodules  have  been  regarded  as  vary- 
ing in  shape  from  time  to  time,  being  but  transient  accumulations  of  lym- 
phocytes. 

The  capsule  of  the  spleen  is  divided  into  two  layers.  The  outer  is 
the  tunica  serosa  and  the  inner,  the  tunica  albuginea.  The  serosa  consists 
of  the  peritoneal  mesothelium  which  covers  the  spleen  except  around  its 
hilus,  and  of  the  underlying  connective  tissue.  The  albuginea  is  a  dense 
layer  of  connective  tissue,  containing  elastic  networks  and  smooth  muscle 
fibers.  Similar  tissue  is  found  in  the  trabeculae.  The  muscle  elements 
are  less  numerous  in  the  human  spleen  than  in  those  of  many  animals. 
By  contraction  they  force  blood  from  the  pulp  and  cause  the  circulation  to 
follow  more  definite  channels.  When  they  are  paralyzed  the  pulp  becomes 
filled  with  the  blood  corpuscles. 

The  nerves  of  the  spleen,  from  the  right  vagus  and  the  coehac  sym- 
pathetic plexus,  are  medullated  and  non-medullated  fibers,  chiefly  the 
latter.  They  form  plexuses  around  the  blood  vessels  (Fig.  184)  and  send 
jfibers  into  the  pulp.     Besides  supplying  the  muscles  of  the  vessels  and 


164 


HISTOLOGY, 


trabeciilae,  some  of  them  are  thought  to  have  free  sensory  endings.  Lym- 
phatic vessels  are  said  to  occur  in  the  capsule  and  trabeculae,  but  not  in 
the  pulp  or  nodules  of  the  spleen. 

The  spleen  is  a  large  organ,  without  obvious  subdivisions.  On  its  surface, 
when  fresh,  there  is  a  mottled  effect  due  to  areas  bounded  more  or  less  definitely 
by  trabeculae.     Such  areas,  about  i  mm.  in  diameter,  have  been  described  by 


Surface  blackened  by 
precipitate  of  silver. 


•^      ----    Xerv 


es  of  the  pulp. 


spleen  nodule 


Ner\-e  branches 

for  the  arterial 

wall. 


Small  nerve 
bundle. 


Branches  for  the         /     / 
arterial  wall.  ,:'■ ^^ 


Fig.  1S4.— GoLGi  Preparation  of  the  Spleen  of  a  Moise.     X  85. 

The  boundary  between  the  spleen  pulp  and  the  lymphoid  tissue  is  indicated  by  a  dotted  line. 

The  nerves  are  chiefly  in  the  wall  of  an  artery. 


Professor  Mall  as  'lobules'  and  he  states  that  they  "can  easily  be  seen  on  the 
surface  of  the  organ  or  in  sections."  A  lobule  as  he  describes' it,  has  a  central 
artery,  and  its  base  is  where  the  l)-mphoid  sheath  of  the  artery  terminates.  There 
are  veins  in  the  trabeculae,  often  three,  at  the  periphery.  A  lobule  is  composed 
of  some  ten  structural  (or  histological)  units,  imperfectly  separated  from  one 
another  by  branches  of  the  trabeculae.  Each  unit  contains  a  central  terminal 
artery  (branches  of  the  lobular  arter}-)  and  has  peripheral  veins  (branches  of 
those  about  the  lobule).     Apparently,  therefore,  the  lobules  shown  in  the  dia- 


THE    ENTODERMAL    TRACT. 


l6  = 


gram,  Fig.  i8i,  except  along  its  lower  border,  represent  groups  or  pairs  of  Mall's 
lobules.  Professor  Stohr  notes  that  "  a  division  into  lobules  in  the  interior  of 
the  spleen  is  impossible."  The  arrangement  of  lobules  at  the  periphery  suggests 
an  ill-defined  cortex.  Lobes  have  also  been  described,  corresponding  with  the 
main  branches  of  the  splenic  artery,  but  the  lobes  are  not  generally  recognized. 
The  spleen  may  present  inconstant  subdivisions,  which  sometimes  produce 
detached  portions  called  accessory  spleens. 


THE  ENTODERMAL  TRACT. 
Development  of  the  mouth  and  pharynx. 

In  a  previous  section  the  early  development  of  the  pharyngeal  pocket 
of  entoderm  has  been  described  and  illustrated  (Fig.  20).  This  'pharynx' 
of  the  young  embryo  is  to  produce  the  fore  part  of  the  intestinal  tract  in- 
cluding the  pharynx,  oesophagus,  and  stomach  of  the  adult.  Its  anterior 
extremity  encounters  the  ectoderm  at  the  bottom  of  a  depression.  The 
ectoderm  and  the  entoderm  there  fuse  to  make  the  oral  plate  (Fig,  185), 
which  becomes  thin,  ruptures,  and  disappears.  Just  anterior  to  the  plate, 
in  the  median  line,  the  ectoderm  sends  a  gland - 
like  projection  toward  the  brain.  It  branches 
and  becomes  detached  from  the  oral  ectoderm, 
lying  in  the  sella  turcica  of  the  adult.  It  is 
known  as  the  anterior  lobe  of  the  hypophysis,  and 
it  vidll  be  described  with  the  brain,  from  which 
the  posterior  lobe  develops.  The  ectoderm  in 
front  of  the  oral  plate  forms  also  the  epithelium 
of  the  lips  and  of  the  peripheral  part  of  the  mouth 
including  the  enamel  organs,  as  has  already  been 
described.  No  line  of  separation  between  the 
ectoderm  and  entoderm  can  be  found  in  the  adult. 

The.  entoderm  of  the   mouth  and  pharynx 
constitutes  the  epithelium  lining  a  broad  cavity 

flattened  dorso-ventrally.  It  produces  a  succession  of  paired  lateral  out- 
pocketings  which  meet  corresponding  ectodermal  depressions.  Ectoderm 
and  entoderm  fuse  where  these  meet,  making  plates  similar  to  the  oral 
plate,  and  in  fishes  these  rupture  to  produce  the  branchial  clefts  (gill 
clefts).  Their  arrangement  in  a  young  dog-fish  is  shown  in  Fig.  186. 
The  mouth,  m,  leads  into  a  cavity,  the  pharynx,  which  opens  freely 
on  the  outer  surface  of  the  fish  through  five  gill  clefts,  g.c.  It  also 
opens  to  the  surface  through  the  spiracle,  sp,  a  structure  similar  to  the  gill 
clefts  but  anterior  to  them  and  having  a  more  dorsal  aperture.  Gill  clefts 
and  spiracle  occur  on  both  sides  of  the  fish.     In  mammalian  embryos 


Fig.  185. — Diagram  showing 
THE  Relations  between 
Ectoderm  and  Entoderm 
in  the  Mouth  of  a  Mam- 
malian Embryo. 

a.  I.,  and  p.  I.,  Anterior  and 
posterior  lolaes  of  tiie  hypo- 
physis ;  m.  t.,  medullary 
tube;  ph.,  pharynx;  o.  p., 
oral  plate  ;  x.  and  y.,  ecto- 
derm which  produces  the  lip 
and  teeth  of  the  lower  and 
the  upper  jaw  respectively. 


i66 


HISTOLOGY. 


these  structures  are  rudimentary;  if  their  closing  plates  ever  rupture  they 
are  soon  restored  so  that  permanent  openings  from  the  pharynx  on  the  side 
of  the  neck  are  not  found.  In  mammals  fojr  clefts  are  indicated  by  ecto- 
dermal depressions  as  shown  in  Fig.  187.     Posterior  to  the  mouth  is  the 


Fig.  1S6.— Hkad  of  a  Voung  Dog-fish.  Fig.    1S7.— Head    of    Human    Embryo  of 

g.  C,  Gill  cleft  ;  m..  mouth  ;  n.,  nasal  pit;  sp.,  spiracle.  10  mm. 

C.  S.,  cervical  sinus;  g.  c.  2.,  second  gill  cleft; 
h.,  liyoid  arch  ;  m.,  mouth;  md.,  mandibu- 
larprocess;  n.,  nasal  pit ;  sp.,  auditory 
(spiracular)  groove. 

auditory  (spiracular)  groove,  which  is  counted  as  the  first  gill  cleft;  it  gives 
rise  to  the  external  aurhtory  meatus  and  around  it  the  auricle  develops. 
The  ectodermal  groove  connected  with  the  second  gill  cleft  disappears. 
Those  of  the  third  and  fourth  form  a  single  deep  depression  on  the  side  of 

the  neck,  called  the  cervical  sinus,  which 
persists  only  in  pathological  cases,  and  is  a 
source  of  branchial  cysts. 

The  entodermal  portion  of  the  gill 
clefts  in  a  mammal  is  shown  in  Fig.  i88. 
The  pharynx  opens  to  the  exterior  at  the 
mouth,  m,  and  divides  posteriorly  into  the 
trachea,  tr,  and  oesophagus,  oe.  In  the 
median  dorsal  hne  it  gives  rise  to  the  an- 
terior lobe  of  the  hypophysis,  cut  off  at 
a.  L,  and  in  the  median  ventral  line  to  the 
thyreoid  gland,  t.  The  latter  grows  down 
through  the  hind  part  of  the  tongue,  acquiring  a  position  in  front  of  the 
trachea.  Its  branching  terminal  part  becomes  separated  from  its  outlet, 
the  foramen  caecum,  by  the  obhteration  of  its  duct  (called  the  thyreoglossal 
duct).  Thus  the  thyreoid  gland  is  a  detached  clump  of  endothelial  tubules 
in  front  of  the  trachea.  The  entodermal  portions  of  the  gill  clefts  are  four 
paired  lateral  outpocketings.  The  first  (i)  extends  to  the  auditory  groove 
in  the  ectoderm,  and  it  becomes  in  the  adult  the  auditory  tube  (Eustachian 
tube)  and  the  middle  ear.  It  will  be  further  described  with  the  sense 
organs.  The  second  pharyngeal  pouch  (2)  disappears  except  as  it  forms 
a  depression  in  the  lower  part  of  which  the  palatine  tonsil  develops.     Its 


Fig.  188. — Diagram    of  the    Pharynx 

of  a  Mammalian  Embryo. 
a.  I.,  Anterior  lobe  of  the  hypophysis;  f.  c., 

foramen   caecum;    m.,    mouth;     oe., 

oesophagus;  p.  b.,  postbranchial  body; 

t..   thyreoid;  th..  thymus;  tr.,  trachea; 

1,  2,  3,  4,  the  pharyngeal  pouches. 


GILL    CLEFTS. 


167 


epithelium  may  become  that  of  the  tonsil.  The  upper  portion  of  the  de- 
pression made  by  the  second  pouch  probably  becomes  the  pharyngeal 
recess  [fossa  of  Rosenmiiller].  The  third  pouch,  near  where  it  meets  the 
ectoderm,  sends  a  tubular  diverticulum  {th)  down  the  neck  behind  the 
thyreoid  gland;  it  continues  into  the  thorax,  lying  ventral  to  the  arch  of  the 
aorta  (Fig.  189).  The  diverticulum  loses  its  lumen  and  becomes  detached 
from  the  pharynx;  it  forms  the  thymus.  Besides  this  elongated  structure, 
the  third  pouch  produces  a  rounded  clump  of  cells  which  becomes  sepa- 
rated from  the  upper  or  anterior  end  of  the  thymus.  This  nodulus  thymi- 
cus  has  been  said  to  produce  the  glomus  caroticum;  but  the  latter  is  now 
generally  regarded  as  a  vascular  mesenchymal 
structure.  The  nodulus  thymicus  has  also  been  said 
to  form  a  small  body  attached  to  the  posterior  sur- 
face of  the  thyreoid  gland  in  the  adult,  and  called 
the  parathyreoid  gland.  The  origin  of  the  parathy- 
reoid  glands,  of  which  there  may  be  four  in  man,  two 
on  either  side,  is  still  uncertain;  and  the  fate  of 
the  nodulus  thymicus  is  obscure.  The  fourth 
pharyngeal  pouch  (4)  soon  becomes  Y-shaped  by 
union  with  the  postbranchial  body  (p.b.).  The  latter 
is  an  independent  outgrowth  of  the  pharynx,  aris- 
ing near  the  fourth  pouch,  and  considered  either  a 
rudimentary  fifth  pouch,  or  a  structure  not  related 
to  the  pouches.  It  elongates  and  fuses  with  the  thy- 
reoid gland,  from  the  tissue  of  which  it  is  scarcely 
to  be  distinguished.  Embryologists  differ  as  to  whether  it  forms  any  of 
the  adult  thyreoid  gland.  The  fourth  pouch  itself  produces  a  nodule  of 
tissue  which  has  been  said  to  form  the  anterior  pair  of  parathyreoid  glands, 
but  its  fate  is  still  uncertain. 

Since  the  derivatives  of  the  first  pouch  are  to  be  described  with  the  ear, 
it  remains  to  consider  the  palatine  tonsils,  as  related  with  the  second  pouch; 
the  thymus,  as  derived  from  the  third;  the  thyreoid,  from  the  floor  of  the 
mouth  and  from  the  postbranchial  bodies;  and  the  parathyreoid  glands 
from  the  third  ( ?)  and  fourth  ( ?)  pouches. 


3L,<y 

Fig.  189. 
The  thymus,  th.,  and  thyre- 
oid, t., of  29  mm.  human 
embryo;  p.,  parathyre- 
oid gland  [derived  from 
the  3d  pouch  (?)];  p.g., 
parathyreoid  gland  [de- 
rived from  the  4th  pouch 
(?)];  p.  I.,  pyramidal 
lobe  of  the  thyreoid ; 
ao.,  aorta  ;  v.,  vena  cava 
superior.  (After  Ver- 
dun.) 


Palatine  tonsils. 
The  palatine  tonsils  are  two  rounded  masses  of  lymphoid  tissue,  one 
on  either  side  of  the  throat,  between  the  arches  of  the  palate.  They  are 
covered  by  the  mucous  membrane  or  tunica  mucosa,  which  throughout  the 
digestive  tract  consists  of  several  layers.  The  entodermal  epithelium  rests 
on  a  connective  or  reticular  tissue  layer,  the  tunica  propria.     A  structure- 


i68 


HISTOLOGY. 


less  basement  membrane  beneath  the  epithelium  is  called  the  membrana 
propria.  The  epithehum,  membrana  propria,  and  tunica  propria  together 
form  the  mucous  membrane.  Beneath  it,  and  sometimes  not  clearly  sepa- 
rable from  the  tunica  propria,  is  the  suhmucous  layer,  or  tela  suhmucosa. 
It  is  a  vascular  connective  tissue  by  which  the  mucous  Membrane  is  at- 
tached to  underlying  muscles  or  bones.  All  of  the  layers  named  are  in- 
volved in  the  tonsils  which,  however,  are  essentially  lymphoid  accumula- 
tions in  the  tunica  propria. 

The  epithelium  of  the  palatine  tonsils  is  a  stratified  epithelium  of 
many  layers,  with  flattened  cells  on  its  smooth  free  surface,  and  columnar 


/j  >.:ii;v'/i" 


X'.     i-'^^^'^->   r-:---^;;v^SS* 


^^: 


^:x 


/ 

Fig.  190. — \'ertical  Section  of  a  Human  Palatine  Tonsil. 
a,  Stratified  epithelium;  b,  basement  membrane;  c,  tunica  propria;  d,  trabeculae ;  e,  diffuse  lymphoid 
tissue;   f,  nodules;    h,  capsule;    i,  mucous  glands;    k,   striated   muscle;    I,  blood    vessel;    q,   pits. 
(From  Radasch.) 


cells  beneath.  Its  attached  surface  is  invaded  by  connective  tissue  ele- 
vations or  papillae  so  that  it  appears  wavy  in  sections  (Fig.  190).  The 
stratified  epithelium  lines  from  ten  to  twenty  almost  macroscopic  depres- 
sions called  tonsillar  pits  or  fossulae  (crypts).  These  are  irregularly  cylin- 
drical and  sometimes  branched.  Many  lymphocytes  penetrate  between 
the  epithelial  cells  and  escape  from  the  free  surface  into  the  saliva,  to  be- 
come 'salivary  corpuscles.'  In  places  the  tonsillar  epithelium  is  so  full  of 
lymphocytes  as  to  appear  disintegrated.  In  the  reticular  tissue  of  the 
tunica  propria,  especially  around  the  pits,  there  are  many  lymph  nodules, 
some  of  which  are  well  defined  with  germinative  centers,  but  many  others 
are  fused  in  indefinite  masses.  The  lymphoid  tissue  forms  the  bulk  of  the 
tonsil. 


PALATINE    TONSILS.  1 69 

The  submucous  layer  forms  a  capsule  for  the  organ,  into  which  it 
sends  trabecular  prolongations.  It  contains  many  blood  and  lymphatic 
vessels,  together  with  the  secreting  portions  of  mucous  glands,  and  the 
branches  of  the  glossopharyngeal  nerve  and  of  the  spheno-palatine  gang- 
lion which  supply  the  tonsil.  Some  of  the  small  glands  empt}'  into  the 
pits  but  most  of  their  ducts  terminate  in  the  mucous  membrane  surround- 
ing the  tonsil.  They  resemble  other  mucous  glands  of  the  mouth  which 
are  to  be  described  presently.  Beyond  the  submucosa  is  striated  muscle, 
belonging  to  the  arches  of  the  palate  and  to  the  superior  constrictor  of  the 
pharynx. 

Except  that  the  palatine  tonsils  he  in  depressions  which  correspond 
in  position  with  the  second  pharyngeal  pouches,  they  afford  no  evidence  of 
their  branchial  relations.  Only  their  epithehum  is  entodermal.  The 
lymphoid  tissue  is  mesenchymal.  In  these  respects  the  palatine  tonsils 
resemble  the  median  lingual  tonsil  which  forms  the  posterior  part  of  the 
tongue  (see  page  184)  and  the  more  diffuse  median  pharyngeal  tonsil  on  the 
dorsal  waU  of  the  nasophar}Tix  between  the  openings  of  the  auditory  tubes. 
Irregular  enlargements  of  the  latter  may  obstruct  the  inner  nasal  openings, 
producing  the  'adenoids'  of  chnicians  (the  adjective  adenoid  being  s^mony- 
mous  with  lymphoid).  The  pits  of  the  pharyngeal  tonsil  are  smaller  than 
those  of  the  palatine. 

Thymus. 

The  thymus  arises  from  the  two  tubular  prolongations  of  the  third 
pharyngeal  pouches,  which  meet  in  the  median  hne  as  shown  in  Fig.  189, 
and  become. bound  together  by  their  connective  tissue  coverings.  The 
lumen  is  lost,  and  the  cells  prohferate.  They  form  a  broad,  flat,  bilobed 
mass  with  a  tapering  prolongation  up  either  side  of  the  neck.  The  bulk 
of  the  organ  is  in  the  thorax,  beneath  the  upper  part  of  the  sternum.  At 
birth  it  weighs  generally  between  5  and  15  grams  (about  half  an  ounce), 
and  is  relatively  a  large  organ.  It  increases  in  size  and  weight  for  some 
years  after  birth,  probably  until  puberty,  and  then  slowly  atrophies.  At 
15  years  it  is  said  to  weigh  from  40-50  grams.  It  is  considered  an  active 
organ  even  to  the  fortieth  year,  losing  its  functions  with  beginning  old  age 
(50-60  years).  Then  it  becomes  fibrous  and  fatty.  The  importance  of 
the  th}Tnus  has  apparently  been  underestimated. 

The  thymus  is  subdivided  by  connective  tissue  layers  into  lohes  from 
4  to  II  mm.  in  diameter,  and  these  are  similarly  subdivided  into  lobules 
of  about  one  cubic  milhmeter  each.  On  either  side  all  the  lobules  are 
attached  to  a  cord  of  medullary  substance,  1-3  mm.  in  diameter,  as  may 


I70 


HISTOLOGY. 


be  seen  if  the  gland  is  pulled  apart.     The  medullary  substance  extends 
from  the  cord  into  the  lobules  (Figs.  191  and  192)  where  it  is  partially ^sur- 


Coniiective 
tissue. 


Transverse 
section  of 
/^.\  blood  vessel. 


_  J  Medullarv  cord. 


Fig.  191.— Fro.m  a  Cross  Section  of  thk  Thymus  of  a  Child,  i  Year  and  9  Months  Old.     X  21. 


Cortical  substance. 


mmii. 


Medullarv  substance. 


Blood  vessel. 


Thymic  ; 

corpuscle.   "^^   '  ^■. 

.M. 


.iM^»j>. 


rounded  by  a  denser  cortical  substance.     In  places  the  medulla  is  in  con- 
tact with  interlobular  connectiYC  tissue.     The  cortex  and  medulla  are  not 

sharply    separated   from 
one  another. 

The  cells  of  the  thy- 
mus have  been  variously 
interpreted.  According 
to  a  recent  investigation 
(by  Dr.  E.  T.  Bell)  the 
thymus  is  at  first  a  com- 
pact mass  of  entodermal 
cells.  By  vacuohzation 
the  cells  form  a  reticulum, 
and  certain  of  them  be- 
come lymphocytes.  The 
lymphocytes  pass  into 
the  cortex  where  they  are 
most  abundant,  and  enter 
the  vessels.  The  lym- 
phoid transformation  of 
the  thymus  "is  noticeable 
in  pigs  of  3.5  cms.  and  is  well  advanced  at  4.5  cms."  It  has  already  been 
stated  that  lymphocytes  are  first  recognizable  in  the  blood  and  in  the 
lymph  glands  of  pigs  of  8  cms.     The  possible  first  appearance  of  lympho- 


Tangential  sections  of  lobules. 


Fig.  192.— Part  of  a  Section  of  the  Thymus  from 
A  5  Months'  Human  Fetus,    x  50. 


THYMUS. 


171 


cytes  in  the  thymus  and  their  origin  from  entoderm  are  of  great  interest. 
That  the  thymus  cells  are  lymphocytes,  however,  is  denied  by  Professor 
Stohr  who  regards  the  cortex  as  composed  of  round  entodermal  cells  de- 
ceptively similar  to  lymphocytes,  and  as  forming  a  degeneration  zone  of 
'thymus  tissue.  Of  true  leucocytes  in  the  thymus  he  says, — "  In  the  places 
where  the  medulla  is  directly  in  contact  with,  the  surrounding  .connec^ 
tive  tissue — and   such  places  become  constantly  larger  and  more  numer- 


Vein. 


Connective  tissue 


Thymic  corpuscle 


'    "'^Jvi 


V 


*w^^i 


Entering  Medullary 

leucocytes.        substance. 


Fig.  193. — Part  of  a  Section  of  the  Thymus  of  a  Child  .-^t  Birth.    X  50. 

ous  as  the  organ  grows — many  leucocytes  wander  into  the  medulla;  they 
lie  in  the  connective  tissue  surrounding  the  medulla  but  not  in  that 
around  the  cortex  (Fig.  193)."  Still  another  view  is  that  the  cortex 
consists  of  reticular  tissue  of  mesenchymal  derivation,  containing  l}Tn- 
phocytes  arising  like  those  in  lymph  glands.  The  original  entodermal 
pouch  is  thought  to  become  surrounded  by  dense  mesenchyma  and  to  form 
but  an  insignificant  part  of  the  medalla.  The  nature  of  the  thymus  then 
must  still  be  considered  obscure. 


172  HISTOLOGY. 

Not  only  lymphocytes,  but  other  leucocytes,  eosinophilic  cells,  and 
multinu clear  giant  cells  have  been  found  in  the  medulla.  Erythroblasts 
are  said  to  occur  in  its  outer  portion  and  in  the  cortex.  The  thymus  there- 
fore is  considered  a  blood  forming  organ.  In  ordinary  sections  it  resembles 
a  lymph  gland,  from  which  it  may  be  distinguished  by  the  presence  of 
thymic  corpuscles  [Hassall's  corpuscles]  in  its  medulla.  These  corpuscles 
are  found  exclusively  in  the  medulla  of  the  thymus.  They  are  rounded 
bodies,  at  first  few  in  number  and  small  (12-20  i'.  in  diameter),  but  they 
increase  rapidly  in  size  (to  a  diameter  of  180  //)  and  new  ones  are  con- 
stantly forming.  At  birth  they  are  numerous,  varying  in  size  as  shown 
in  Fig.  193.  To  produce  them,  the  nucleus  and  protoplasm  of  a  reticular 
tissue  cell  (entodermal)  are  said  to  enlarge.  The  nucleus  loses  its  staining 
capacity  by  changes  in  its  chromatin,  and  a  layer  of  deeply  staining  hyahne 

Degenerated  epithelial  cells 

Flat  epithelial  cells. 
Dearetierated  nucleus. 


Fig.  194.— Thv.mic  Corpuscles,  in  Sectio.n,  from  a  ]M.\n  23  Ve.\rs  Old.    >(  360. 

substance  develops  in  the  protoplasm.  This  increases  until  it  fills  the  entire 
cell,  often  being  arranged  in  concentric  layers.  The  nucleus  becomes 
obliterated.  Neighboring  cells  are  concentrically  compressed  by  the  en- 
largement of  this  structure,  and  by  hyaline  transformation  they  may  be- 
come a  part  of  the  corpuscle.  The  larger  corpuscles  are  due  to  a  fusion 
of  smaller  ones,  or  to  hyaline  changes  occurring  simultaneously  in  a  group 
of  cells.  The  central  portion  of  a  corpuscle  may  become  calcified.  Some- 
times it  is  vacuolated,  containing  fat.  The  hyahne  substance  may  respond 
to  mucus  stains,  but  generally  it  does  not;  it  has  been  considered  similar 
to  the  'colloid'  of  the  thyreoid  gland.  Leucocytes  are  said  to  become  im- 
bedded in  the  corpuscles  or  to  enter  them  and  assist  in  their  disintegration. 
Thymic  corpuscles  have  been  regarded  as  degenerative  products  of  the  en- 
todermal epithelium;  as  concentric  connective  tissue  masses ;  and  as  blood 
vessels  with  thickened  walls  and  obliterated  cavities.     Injections  show 


THYREOID   GLAND. 


17: 


that  they  are  not  connected  with  the  blood  vessels.  Although  they  have 
recently  been  described  as  active  constituents  of  the  thymus  they  are  gener- 
ally regarded  as  degenerations. 

The  arteries  of  the  thymus  enter  it  along  the  medullary  strand  and 
extend  between  the  cortex  and  medulla,  sending  branches  into  both  but 
chiefly  into  the  cortex.  The  cortical  braxiches  empty  into  veins  between 
the  lobules;  the  others  into  those  within  the  medulla.  There  are  many 
interlobular  lymphatic  vessels  beginning  close  to  the  surface  of  the  gland, 
and  accompanying  the  blood  vessels.  There  is  nothing  in  the  thymus  to 
correspond  with  a  lymph  sinus.  The  nerves,  chiefly  sympathetic  fibers, 
with  some  from  the  vagus,  terminate  on  the  vessels;  a  very  few  have  free 
endings  in  the  medulla. 

Thyreoid  gland. 

The  thyreoid  gland  is  a  median,  entodermal  downgrowth  from  the 
tonguej_^  thyreoglossal  ^duct  becomes  obliterated,  leaving  the  foramen 
caecum  to  mark  its  former  outlet.  The  downgrowth  is  joined  by  cells 
from  the  postbranchial  bodies,  which  fuse  with  it.  This  entire  structure 
comes  to  he  beside  and  in  front  of  the  upper  part  of  the  trachea.  It  con- 
sists of  two  lateral  lobes,  each  about  two  inches  long  and  an  inch  wide, 
connected  by  an  isthmus,  about  half  an  inch  wide,  which  crosses  the  median 
line  ventral  to  the  second  and  third  tracheal  rings.  An  unpaired  pyram- 
idal lobe  extends  from  the  isthmus  or  adjacent  part  of  the  lateral  lobe 
toward  the  tongue  (Fig.  189).  Irregular  detached  portions  of  the  gland 
such  as  occur  especially  along  the  course  of  the  thyreoglossal  duct,  are 
called  accessory  thyreoid  glands. 

The  prohferating  mass  of  entodermal  cells  forms  at  first  a  network  of 
sohd  cords.  This  becomes,  separated  into  small  masses  within  each  of 
which  a  lumen  may  appear.  The  lumen  enlarges  and  becomes  spheroidal ; 
the  entodermal  cells  which  surround  it  form  a  simple  epithelium,  either 
columnar,  cuboidal,  or  flat.  Flat  cells  are  said  to  occur  especially  in  old 
age,  low  columnar  or  cuboidal  cells  being  usually  found.  The  mature 
thyreoid  gland  consists,  therefore,  of  rounded,  closed  spaces,  or  follicles, 
bounded  by  a  simple  entodermal  epithelium  (Fig.  195).  The  folhcles 
vary  greatly  in  diameter.  Generally  they  are  rounded,  but  sometimes 
they  are  elongated,  and  occasionally  they  branch  or  communicate  with  one 
another.  Among  them  are  cords  or  clumps  of  cells  which  have  not 
acquired  a  lumen. 

Within  the  folhcles,  and  forming  the  most  conspicuous  feature  of  the 
thyreoid  gland  in  ordinary  sections,  is  a  hyahne  material  which  stains 
deeply  with  eosine  and  is  named  'colloid.'     Its  chemical  nature  is  unde- 


174 


HISTOLOGY. 


termined.  The  hyaline  material  in  the  thymic  corpuscles,  the  hypophysis, 
and  in  the  coagulum  in  the  cervical  blood  and  lymphatic  vessels,  has  also 
been  designated  colloid.  In  sections  of  the  thyreoid  gland  it  usually  does 
not  fill  the  follicle  but  has  contracted,  producing  a  spiny  border.  Gran- 
ules, vacuoles,  detached  cells,  leucocytes,  and  crystalloid  bodies  may 
be  found  in  it.  It  is  a  product  of  the'epithehal  cells,  in  the  protoplasm  of 
which  similar  material  has  been  detected.  It  has  been  said  that  it  is  trans- 
ferred to  the  blood  and  lymphatic  vessels. 

As  has  been  learned  by  experiment,  the  thyreoid  gland  produces  an 
internal  secretion  which  is  essential  for  the  normal  growth  and  develop- 
ment of  the  body.  It  is,  however,  not  known  whether  this  secretion  leaves 
the  basal  or  free  surface  of  the  thyreoid  epithehum,  and  its  relation  to  the 
colloid  material  is  not  clear.     The  finding  of  two  sorts  of  thyreoid  cells, 


Colloid  substance. 


Epithelium l^pj 


Tangential  section  of  a 
tubule;  the  epithelium 
viewed  from  the  surface. 


Pubule  in  transverse 
section. 


Connective  tissue. 


Fig.  195.— Section  of  a  Lobli.k  of  thk  Thyreoid  Gl.and  from  .an  Adult  M.j 


one  of  which  produces  colloid,  and  the  other  does  not,  lacks  confirmation. 
The  cells  may  exhibit  refractive,  secretory  granules  which  are  larger  and 
coarser  toward  the  free  surface.  In  certain  animals  other  granules  of 
fatty  nature  have  been  found,  especially  near  the  basal  surface.  Since  the 
terminal  bars  are  said  to  be  deficient  at  the  angles  where  the  epithelial  cells 
meet,  an  opportunity  is  afforded  for  the  contents  of  the  folhclcs  to  pass  out 
between  the  epithelial  cells  to  the  vascular  tunica  propria. 

The  thyreoid  follicles  are  surrounded  by  loose  elastic  connective  tissue, 
said  to  be  reticular  near  the  folhcles,  which  contains  \evx  many  blood  and 
lymphatic  vessels.  Denser  connective  tissue  forms  a  capsule  and  lobular 
partitions.     The  nerves  from  the  cervical  sympathetic  gangha  form  peri- 


PARATHYREOID    GLAXD5 


vascular  plexuses,  and  pass  to  the  follicles. 
between  the  epithehal  cells. 


A  few  have  been  found  to  end 


Parathyreoid  gl.axds. 
It  is  generally  stated  that  there  are  four  parath}Teoid  glands  in  man, 
the  anterior  or  upper  pair  being  derived  from  the  fourth  entodermal 
pouches,  and  the  posterior  or  lower  pair  from  the  nodiihis  thymicus  of  the 
third  (Tig.  189).  Although  they  have  been  repeatedly  investigated,  their 
origin  is  not  yet  established.  In  the  adult  they  are  round  or  oval  bodies, 
said  to  measure  from  3  to  13  mm.,  found  on  the  dorsal  or  tracheal  surface 
of  the  th}Teoid  gland.  They  may  be  imbedded  in  its  capsule  or  attached 
to  it  by  pedicles.     Sometimes  they  (the  lower  pair?)  are  found  in  the  thy- 


.^ 


^ 


^     © 


)^ 


^ 


/ 


® -®^. 


Fig.  196. — Section  of  a  Human  Parath-s-reoid  Gland.     (Huber.) 


mus.  It  is  not  kno'v\'n  that  two  pairs  always  occur.  The  parath}Teoid 
glands  may  be  lacking  on  one  side,  where  in  other  cases  as  many  as  four 
have  been  recorded.  Both  pairs  possess  a  similar  structure  unhke  that  of 
either  the  thyreoid  gland  or  the  th}Tnus,  but  resembling  the  corresponding 
epithelial  bodies  of  the  lower  vertebrates.  They  consist  of  masses  and 
cords  of  polygonal,  entodermal  cells,  containing  round  nuclei  with  networks 
of  chromatin.  The  protoplasm  is  pale,  "almost  homogeneous"  or  "slightly 
granular,"  sometimes  containing  vacuoles.  CeU.  membranes  are  not  promi- 
nent. Between  these  cells  and  the  large  thin-walled  blood  vessels  which 
pass  among  them  (Fig.  196),  there  is  only  a  very  small  amount  of  connec- 
tive tissue.  A  capsule  surrounds  the  entire  structure.  The  blood  vessels 
are  branches  of  those  which  supply  the  th}Teoid  gland.  little  is  known 
of  the  lymphatics  or  nerves. 


176 


HISTOLOGY. 


Glomus  caroticum. 
The  glomus  caroticum  [carotid  gland]  has  already  been  described  as  a 
knot  of  blood  vessels  at  the  bifurcation  of  the  common  carotid  artery. 
It  is  a  reddish  body  "5-7  mm.  long,  2.5-4  mm.  broad,  and  1.5  mm,  thick." 
Between  its  thin  walled,  dilated  capillaries  there  are  strands  of  polygonal 
cells  said  to  be  chromaflfine  and  prone  to  disintegrate  (Fig.  197).     Many 

nerve  fibers,  medulla- 
ted  and  non-medulla- 
ted,  enter  the  glomus 
and  a  few  multipolar 
gangUon  cells  are  asso- 
ciated with  them.  In 
its  arrangement  of 
cells  and  blood  vessels 
it  resembles  a  para- 
bv.  thyreoid  gland,  and 
also  the  glomus  coccyg- 
eum  which  is  far  re- 
moved from  entoder- 
mal  structures.  Since 
the  nature  of  the 
glomus  caroticum  is 
undetermined,  the 
three  views  regarding 
it  may  be  mentioned. 
First,  it  has  been  con- 
sidered derived  from 
the  nodulus  thymicus,  which  is  now  said  to  form  a  parathyreoid  gland. 
Recently  it  has  been  found  that  the  'carotid  gland'  of  Echidna  comes  from 
the  second  pharyngeal  pouch,  and  the  non-en todermal  origin  of  the  human 
glomus  is  not  beyond  question.  Second,  it  has  been  considered  ganglionic 
or  paraganglionic  in  nature,  so  that  it  is  classed  with  nervous  structures. 
Third,  it  is  considered  essentially  a  vascular  formation,  containing  strands 
of  modified  mesenchymal  cells. 


Fig.  197. — Section  of  a  Part  of  the  Glomus  Caroticum 

OF  Man.     (After  Schaper.) 
b.v.,  Blood  vessels;  e.v.,  efferent  vein;  tr.,  trabecula;  c.t.,  con- 
nective tissue  septum. 


Development  and  Structure  of  the  Tongue. 

The  tongue  consists  of  two  parts,  an  anterior  and  a  posterior,  which 

differ  in  origin  and  adult  structure.     Separating  the  branchial  clefts  from 

one  another  there  are  columns  of  tissue  known  as  branchial  arches.    They 

come  together  in  the  median  ventral  line  to  form  the  floor  of  the  mouth 


TONGUE. 


177 


/// 


v^^ 


fV 


Fig.  19S.— Floor  of  the  Pharynx  of  a  10  mm. 
Human  Embryo. 
I-IV,  Bronchial  arches;  f,  anterior  part  of  the  tongue;  t-, 
second  arch,  joining-  the  posterior  part  of  the  tongue 
toward  the  median  line.  The  thyreoid  gland  is  dotted. 
The  epiglottis  extends  over  the  4th  arch.  (From 
McMurrich,  after  His.) 


as  shown  in  Fig.  198.  In  this  figure  the  upper  jaw  and  roof  of  the  pharynx 
have  been  cut  away;  the  branchial  clefts^are  seen  as  dark  depressions 
bounded  laterally  by  thin 
plates.  The  first  branchial 
arch  (i)  is  between  the  oral 
and  auditory  clefts.  In  the 
median  ventral  line  an  eleva- 
tion (tuberculum'  impar)  arises 
between  this  arch  and  the 
second;  it  becomes  contin- 
uous with  a  larger  elevated 
portion  of  the  mandibular  arch 
to  form  the  anterior  part  of 
the  tongue  (t^).  The  second 
and  third  arches  unite  toward 
the  median  ventral  line  and 
there  produce  the  posterior 
part  of  the  tongue  (t^).  Be- 
tween the  anterior  and  poste- 
rior parts  is  the  opening  of  the  thyreoglossal  duct,  later  the  foramen 
caecum.     The  epiglottis  is  an  elevated  part  of  the  third  arch  separated 

from   the  posterior  part  of  .  the  tongue  by  a 
curved  groove. 

In  the  adult,  Fig.  199,  the  dorsum  of  the 
anterior  part  of  the  tongue  is  covered  with 
papillae.  These  are  chiefly  the  slender  fli- 
form  papillae  and  conical  papillae,  but  knob- 
like forms,  the  fungiform  papillae,  are  scat- 
tered among  them  over  the  entire  surface. 
Near  the  junction  of  the  anterior  and  pos- 
terior parts  of  the  tongue  there  is  a  V  shaped 
row  of  larger  papillae,  generally  6  to  12  in 
number,  called  vallate  papillae.  Their  name 
refers  to  the  deep  narrow  depression  which 
encircles  them.  Behind  the  apex  of  the  V, 
which  is  directed  toward  the  throat,  is  the 
foramen  caecum.  On  either  side  of  the  tongue, 
as  indicated  in  the  figure,  there  are  from  3 
to  8  parallel  vertical  folds  (2-5  mm.  long) 
occurring  close  together;  these  are  the  foliate  papillae.  In  the  foliate  and 
vallate  papillae  the  organs  of  taste  are  most  numerous.     The  under  sur- 


Fig.  199. — The  Upper  Surface  of 
THE  Adult  Tongue. 

C,  Conical  papillae;  ep.,  epiglottis; 
f.,  foliate  papillae;  f.  c,  foramen 
caecum;  ff.,  position  of  the  fili- 
form and  fungiform  papillae;  I., 
lenticular  papillae;  I.  t.,  lingual 
tonsil;  p.  t.,  palatine  tonsil;  v., 
vallate  papillae. 


178 


HISTOLOGY. 


face  of  the  tongue  is  free  from  epithelial  papillae;  its  mucosa  resembles 
that  which  lines  the  mouth.  The  posterior  part  of  the  tongue  contains 
the  lingual  tonsil  and  has  a  nodular  surface  covered  with  soft  epithelium. 
Laterally  there  are  fold-like  elevations  called  lenticular  papillae. 

The  tongue  is  composed  of  a  mucous  membra  ae  (tunica  mucosa) 
and  a  submucous  layer,  together  with  the  underlying  striated  muscle  which 
forms  the  bulk  of  the  organ.     Its  anterior  portion  may  be  described  first. 

The  mucous  membrane  is  characterized  by  the  various  papillae.  The 
filiform  papillae  (Figs.  200  and  201)  are  cylindrical  or  conical  elevations  of 
the  tunica  propria,  each  with  from  5  to  20  secondary  papillae  at  its  upper 
end.  They  consist  of  vascular  fibrillar  connective  tissue  with  numerous 
elastic  fibers  and  are  covered  by  a  thick  stratified  epithelium.     The  outer 

epithelial  cells  are  flat  and  corni- 


Cornified 
process. 


Filiform 
papilla 


.^ 


fied, — that  is  they  have  undergone 
a  hornv  hvahne  deo;eneration, — 
and  several  slender  columns  of 
such  cells  may  extend  beyond 
the  secondary  papillae.  The 
filiform  papillae  are  from  0.7  to 
3.0 mm.  tall.  Fungiform  papillae,. 
(Fig.  201)  are  rounded  elevations 
with  a  somewhat  constricted 
base.  The  entire  outer  surface 
of  their  connective  tissue  core  is 
beset  with  secondary  papillae. 
They  contain  but  little  elastic  tis- 
sue; the  epithelium  is  not  as 
thick  as  in  the  filiform  papillae, 
and  its  outer  cells  are  not  cornified.  In  life,  fungiform  papillae  are  red  since 
their  epithelium  transmits  the  color  of  the  blood  beneath.  Their  height 
varies  from  0.5  to  1.5  mm.  The  vallate  papillae  resemble  broad  fungi- 
form papillae.  They  are  from  i  to  3  mm.  broad  and  i  to  1.5  mm.  tall,  each 
being  surrounded  by  a  deep  groove  (Fig.  202).  Their  coniiective  tissue 
often  contains  longituchnal,  oblic|ue,  or  encircling  smooth  muscle  fibers,  the 
last  named  being  found  near  the  lateral  walls.  Secondary  papillae 
are  confined  to  the  upper  wall.  Occasionally  the  epithelium  sends 
branched  prolongations  into  the  underlying  tissue.  These  may  become 
detached  from  the  surface  and  appear  as  concentric  bulb-like  bodies  such 
as  are  generally  known  as  '  epithelial  pearls.'  There  are  also  branched 
serous  glands  which  grow  down  from  the  epithelium,  having  ducts 
which  open  into  the  deep  grooves  (Fig.  202).     The  foliate  papillae   are 


Fat  cells.  Fascia  linguae.  Mu'.cle. 

Fig.  200.— From  a  Longitl-dinal  Skctio.n   of   thk 
dorslm  of  \  hu.man  tongue.    x  12. 


TONGUE. 


179 


parallel  folds  of  mucous  membrane,  in  the  epithelium  of  which  there 
are  many  taste  buds.  These  structures,  which  occur  also  in  the  lateral  walls 
of  the  vallate  papillae,  require  a  detailed  description. 

Taste  buds  are  round  or  oval  groups  of  elongated  epithelial  cells 
which  extend  from  the  inner  to  the  outer  epithehal  surface;  in  contact  with 
them  the  nerves  of  taste  terminate.  Their  position  in  the  epithelium  is 
shown  in  Figs.  202  and  203.  In  the  fetus  of  from  5  to  7  months  they  are 
more  numerous  than  in  the  adult,  occurring  in  many  fihform  papillae  and 


"^^ 

Cornified  epithelium. 

^„^  Secondary 
_>:■-''          papillae 

:m , 

Secondary        'I 
papillae     —--*-- 
of  a  fungiform        *•- 
papilla. 

Funsiform        ^  of 

papilla.    

& 

'S- 

Oblique  section  ""  _  Filiform 

of  a  filiform    -"-  ;    .  „r,/'    ",    '-'^^    papillae. 


papilla 


"W 


Nerves.  °°^^"'^~  ' --. . ^_,.i.'.;..  ''''"''''-"'  '-A 

Vein. "■-■-  :':-"-  -  ^.    :■.   ■ 

Artery.    -"Y" --.  '  .- /:^""'-:.  -\-i 

Fat.  1--.:       '"'"_-_--.    -:  -.,.-'-"■"..,'."  '■_':-..'■'!- Fascia 

t-    "",:---      -.      ■-•     -  "  -.   -'  ,'-L-;- — ■"""5l-'I''j  linguae. 

fevj  ."■."_"-"-•■:_•;,' I    -Tl-.y   '  '-'J Striated  mus- 

^^■-il'ii;'!^;V't'''.  ^  i'T^' ^V.;'-'' '•■   -~   ■>  •  "'\'  cle  fibers. 

Fig.  201. — From  a  Longitudinal  Section  of  the  Human  Tongue.     X  25. 
X,  Epithelium  showing  postmortem  disintegration. 

in  all  the  fungiform,  vallate  and  foliate  forms,  together  with  both  surfaces 
of  the  epiglottis.  They  are  destroyed  with  an  iniiltration  of  leucocytes, 
except  those  on  the  lateral  walls  of  the  vallate  and  foHate  papillae,  small 
numbers  of  those  on  the  anterior  and  lateral  fungiform  papillae,  and  those 
on  the  lar}mgeal  surface  of  the  epiglottis.  In  such  places  they  are  found  in 
the  adult. 

Each  bud  consists  of  two  sorts  of  elongated  epithehal  cells,  among 
which  lymphocytes  are  frequently  seen.  Most  of  the  cells  are  supporting 
cells.     These  may  be  uniform  in  diameter  or  tapering  toward  the  ends. 


i8o 


HISTOLOGY. 


Secondary  papillae.        Taste  bud. 

Vallate  papilla.  ^,' ^  ,  Orifice 

Groove.  '     .'.      ,      ,'  /  of  a 

'   -  /  serous  Small 

/  gland.  papilla. 


Tela 
submucosa. 


O 


Striated 
muscle. 


'*^'^'M.'. 


Muscle  fibers  in  cross 
and  longitudinal  section. 


Nerve  with  Fascia  Mucous 

small  ganglion.        linguae.  gland. 


Fig.  202. — Vertical  Section  of  a  Human  \'allate  Papilla.     X  25. 


-^■-1'- 


Vein. 


Taste  pore. 


ile^c'' 


They  are  sometimes  forked  or  branched  below  and  at  the  free  surface  they 

may  end  in  a  short  conical  pro- 
cess. The  peripheral  halves  of 
the  cells  in  a  taste  bud  converge 
somewhat  like  the  segments  of 
a  melon,  so  that  their  ends  are 
brought  together  in  a  small 
area.  This  area  is  at  the  bot- 
tom of  a  little  pore  or  short 
canal  found  among  the  outer- 
most flat  cells  of  the  epithehum. 
Sometimes  it  is  bounded  by 
the  supporting  cells.  The 
taste  pore  opens  freely  to  the 
surface,  but  in  oblique  sections 
it  may  appear  bridged  as  in 
Fig.  203.  Besides  the  support- 
ing cells  which  are  found  at 

the  periphery  of  the  bud  and  which  terminate  around  or  beneath  the  pore. 


1  unica 
propria. 


Fig.  203.— From  a  Vkki  ical  Section  of  a  Hu.man 
Foliate  Papilla.     X  330. 


TASTE    BUDS. 


i8i 


there  are  more  slender  forms  in  the  interior  of  the  bud,  which  reach  the 
pore.  There  are  also  a  few  fiat  ones  confined  to  the  lower  half  of  the  bud. 
The  taste  cells  are  slender  structures,  being  thickened  to  accommodate  the 
narrow  nucleus.  The  nucleus  is  usually  in  the  middle  or  lower  part  of  the 
cell.  Toward  the  taste  pore  these  cells  generally  taper,  and  they  end  in  a 
stiff  refractive  process  which  is  a  cuticular  formation.  These  processes 
extend  into  the  deeper  part  of  the  pore  but  do  not  reach  its  outlet.  The 
taste  cells  may  have  a  triangular  base,  or  end  bluntly.  Their  protoplasm 
is  darker  than  that  of  the  supporting  cells. 

Taste  bud.  ■      '       ■.;:—-; 


Fibers  between 
the  buds. 


Fibers  within  the  buds. 


Fibers  overlying 
a  bud. 


Connective  tissue. 


Connective  tissue. 
Epithelium. 


—  Xerve. 


Fig.  204. — From  a  Vertical,  Section  of  the  Foli.a.te  Papilla  of  a  Rabbit.    X  220. 


The  nerves  to  the  buds  are  branches  of  the  glossopharyngeus,  asso- 
ciated with  microscopic  sympathetic  ganglia.  These  nerves,  both  medul- 
lated  and  nonmedullated,  make  a  thick  plexus  in  the  submucous  con- 
nective tissue.  The  terminal  branches  probably  end  in  part  in  bulb- 
ous corpuscles,  but  most  of  them,  as  nonmedullated  fibers,  enter  the 
epithelium.  Some  are  found  between  the  taste  buds,  extending  to  the 
outer  epithehal  cells  generally  without  branching  (Fig.  204).  Others 
enter  the  buds,  where  they  divide  into  coarse  varicose  branches  which 
reach  almost  to  the  taste  pore.  They  end  freely,  without  uniting  with 
the  cells  or  anastomosing  with  one  another.     The  terminal  branches  are 


182 


HISTOLOGY. 


chiefly  in  relation  with  the  taste  cells;  to  a  less  extent  they  are  said  to 
ramify  about  certain  of  the  supporting  cells.  The  taste  cells  are  believed 
to  transmit  to  the  nerves  the  stimuli  received  at  the  taste  pore. 

The  tunica  propria  of  the  mucous  membrane,  a  loose  connective  tissue 
layer  containing  fat,  is  not  sharply  separated  from  the  denser  submucosa. 
At  the  tip,  or  apex  linguae,  and  over  the  dorsum,  the  submucosa  is  par- 
ticularly firm  and  thick,  forming  the  fascia  linguae.  Three  sorts  of  glands 
branch  in  the  submucosa  and  may  extend  into  the  superficial  part  of  the 
muscle  layer.  These  are  the  serous  glands  found  near  the  vallate  and 
folhate  papillae;  mucous  glands  occurring  at  the  root  of  the  tongue,  along  its 
borders,  and  in  an  area  in  front  of  the  median  vallate  papilla;  and  the 


Median  section  of  a  nodule. 
Epithelium 


hLI- 


A.i'1 


^- 


Diffuse  lymphoid  tissue. 


-  Epithelium. 


Lymi)h  nodules.  - 


V^-S, 


Periphery 
of  a  nodule. 


Duct  of  a  mucous  gland.  ■ 


jyV''-"'y^rjj. 


}  Tunica  propria. 
Fibrous  capsule. 


Blood  vessel. 


Fig.  205. — From  a  Section'  of  the  Lingual  Tonsil  of  .\n  Adult  Man.     X  20. 

I,  Pit  containing  leucocytes  which  have  infiltrated  its  epithelium  on  the  left  side;   that  on  the  right  is 

almost  intact. 


two  mixed  anterior  lingual  glands,  from  half  an  inch  to  an  inch  long,  each 
of  which  empties  by  five  or  six  ducts  on  the  under  surface  of  the  apex. 
The  appearance  of  these  types  of  glands  will  be  described  in  a  following 
section. 

Blood  vessels  are  numerous  in  the  submucosa  and  form  extensive 
capillary  networks  in  the  tunica  propria  of  both  the  larger  and  the  second- 
ary papillae.  Small  lymphatic  vessels  also  form  a  network  in  the  tunica 
propria  and  this  is  continuous  with  a  coarser  net  in  the  submucosa.  The 
nerves  (sensory)  are  the  terminations  of  the  lingual  branches  of  the  man- 
dibular nerve  anteriorly,  and  of  the  lingual  branches  of  the  glossopharyn- 
geus  posteriorly.  They  contain  nerve  cells  which  are  grouped  in  small 
ganglia,  notably  beneath  the  vallate  papillae.     The  glossophar\Tigeal  end- 


MUSCLES    OF    THE    TONGUE 


183 


ings  in  the  taste  buds  have  been  described.     The  others  terminate  in  bulb- 
ous corpuscles  or  in  free  endings  beneath  or  within  the  epithehum. 

The  muscular  layer  consists  of  interwoven  bundles  of  striated  fibers 
which  are  inserted  into  the  submucosa  or  into  the  intermuscular  connective 
tissue.  Some  of  these  striated  fibers  are  branched.  The  musculature 
of  the  tongue  is  partly  divided  into  right  and  left  halves  by  a  dense  median 
connective  tissue  partition,   the  septum  linguae.     It  begins  low  on  the 

Emigrating  leucocj'tes.        Fragments  of  epithelium. 


Emigrated  leuco- 
cytes. 


y}^ 


Stratified 
epithelium 


i 


*  4r  e  .1  «  A 


o^ 


Lymphoid    tissue 

of  the  tunica 

propria. 

Fig.  206.— From  a  Thin  Section  of  a  Lingual  Tonsil  of  a  Man.     )<  420. 
On  the  left  the  epithelium  is  free  from  leucocytes,  on  the  right  many  leucocytes  are  wandering  through. 

hyoid  bone,  attains  its  greatest  height  in  the  middle  of  the  tongue,  and  be- 
comes lower  anteriorly  until  it  disappears.  It  does  not  extend  clear  through 
the  tongue  since  it  ends  3  mm.  beneath  the  dorsum.  The  muscles  of  the 
tongue  are  partly  vertical  {genioglossus,  hyoglossus,  and  vertkalis  linguae 
muscles),  partly  longitudinal  {styloglossus,  cJiondroglossus,  superior  and 
inferior  longitudinalis  linguae  muscles)  and  partly  transverse  (the  trans- 
versus  linguae  muscle).     The  glosso palatine  muscle  of  the  palatine  group 


184  HISTOLOGY. 

also  enters  the  tongiie.  Some  of  the  muscle  fibers  are  oblique  but  many 
of  the  bundles  cross  at  right  angles.  In  the  connective  tissue  between 
them,  medullated  nerves  are  abundant.  Some  of  these  are  sensory  nerves 
to  the  mucosa  but  many  are  the  lingual  branches  of  the  hypoglossal  nerve 
which  supply  all  the  tongue  muscles  except  the  inferior  longitudinal;  that 
one  is  supplied  by  fibers  from  the  chorda  tympani.  Sensory  spindles 
have  been  found  in  the  lingual  muscles. 

The  posterior  part  of  the  tongue  is  occupied  by  the  lingual  tonsil, 
this  term  being  a  collective  designation  for  a  considerable  number  of 
rounded  masses  of  lymphoid  tissue.  Each  of  these  is  from  i  to  4  mm.  in 
diameter,  and  is  situated  in  the  tunica  propria  so  that  it  causes  a  low, 
macroscopic  elevation  of  the  epithelium.  In  the  center  of  the  elevation 
there  is  a  punctate  depression,  or  pit,  lined  with  stratified  epithehum. 
Around  it  the  lymphoid  tissue  is  partly  separable  into  nodules  with  germi- 
native  centers  (Fig.  205).  The  entire  lymphoid  structure  is  bounded  by 
a  sheath  of  connective  tissue.  Numerous  lymphocytes  enter  the  epithelium, 
and  pass  between  its  cells  to  the  free  surface  where  they  escape  into  the 
saliva.  The  temporary  disintegration  of  the  epithelium,  due  to  this  cause, 
is  shown  in  Fig.  206.  In  all  these  details  the  lingual  tonsil  is  essentially 
hke  the  palatine  tonsils. 

Mouth  and  Pharynx. 

The  fining  of  the  mouth,  like  the  covering  of  the  tongue,  consists  of 
epithehum,  tunica  propria,  and  submucosa.  At  the  lips  toward  the  line 
of  transition  from  skin  to  mucous  membrane,  hairs  disappear  from  the 
skin.  The  epithehum  becomes  abruptly  thicker  but  more  transparent  as 
it  crosses  the  fine.  Its  outer  cells  are  still  cornified,  but  they  are  not  so  flat 
and  compactly  placed  as  in  the  skin.  The  deeper  cells  appear  vesicular. 
Within  the  mouth,  except  on  the  tongue,  cornified  cells  are  absent.  Gran- 
ules of  the  refractive  horny  substance,  keratohyalin,  are  said  to  occur  in 
the  outer  cells,  even  in  the  oesophagus.  The  outer  surface  of  the  epithelium 
is  smooth,  but  its  under  surface  is  indented  by  many  connective  tissue 
papillae,  which  are  particularly  long  and  slender  in  the  lips  (Fig.  207)  and 
gums.  CiHa  occur  on  the  epithelium  in  the  highest  part  of  the  nasal 
pharynx,  and  in  the  fetus  over  the  oral  part  also,  and  even  in  the  oesoph- 
agus.   They  persist  only  in  the  nasal  pharynx. 

The  tunica  propria,  as  is  generally  the  case  in  the  digestive  tract,  has 
few  elastic  fibers.  Some  of  its  tissue  is  reticular  and  in  this,  lymphoid 
accumulations  are  frequent ;  they  may  extend  into  the  submucosa.  On  the 
oral  surface  of  the  soft  palate  there  is  a  layer  of  elastic  tissue  between  the 
propria  and  submucosa.     A  similar  layer  is  found  in  the  oesophageal  end 


GLANDS  OF  THE  ORAL  CAVITY.  185 

of  the  pharynx.  It  increases  in  thickness  upward,  at  the  expense  of  the 
submucosa,  so  that  it  forms  a  thick  layer  in  the  back  of  the  pharynx  in  con- 
tact with  the  muscles,  among  the  fibers  of  which  it  sends  prolongations. 
This  elastic  layer,  as  the  fascia  pharyngohasilaris ,  is  attached  to  the  base  of 
the  skull. 

In  most  of  the  oral  region  there  is  no  shaip  line  of  separation  between 
the  propria  and  the  submucosa.  The  latter  may  be  a  loose  layer  contain- 
ing fat,  and  allowing  considerable  movement  of  the  mucosa,  or,  as  in  the 
gums  and  hard  palate,  it  may  be  a  dense  layer  binding  the  membrane 
closely  to  the  periosteum.     In  the  submucosa  are  the  branches  of  various 


Submucosa. 


Muscle 
fibers . 

Fig.  207.— Vertical  Section  through  thk  AIucols  AlH.MiiKANii  of  the  Lip  of  an  Adult  Man.  X  3°- 

I,  Papilla ;  2,  excretory  duct ;  the  lumen  is  cut  at  only  one  point ;  3,  accessory  gland ;  4,  a  branch  of  the 
excretory  duct  in  transverse  section  ;  5,  gland  bodies  grouped  into  lobules  by  connective  tissue  ;  6,  a 
gland  tubule  in  trans\-erse  section. 

glands.  On  the  inner  border  of  the  lips  and  the  inner  surface  of  the  cheek 
there  are  sebaceous  glands  without  hairs,  which  first  develop  during  puberty. 
This  type  is  described  with  the  skin.  The  other  oral  glands  are  considered 
in  the  following  section. 


Glands  of  the  Oral  Cavity. 

In  the  general  account  of  glands  (page  32)  it  has  been  stated  that 

serous  gland  cells  which  produce  a  watery  albuminoid  secretion  should  be 

distinguished  from  the  mucous  gland  cells  which  elaborate  thick  mucus. 

When  examined  fresh,  serous  cells  are  seen  to  contain  many  highly  refrac- 


i86 


HISTOLOGY. 


tive  granules.  In  fixed  preparations  they  may  appear  dark  and  granular 
(empty  of  secretion)  or  enlarged  and  somewhat  clearer  (full  of  secretion), 
as  shown  in  Fig.  34,  p.  32.  The  round  nucleus  is  generally  in  the  basal 
half  of  the  cell,  not  far  from  its  center  (Fig.  208).  Mucous  cells  when 
fresh  are  much  less  refractive  than  serous  cells.  In  fixed  preparations  they 
are  typically  clear  since  the  large  area  occupied  by  mucous  secretion  stains 

Mail.  Rabbit.  Man. 


Mucous  glands. 


Serous  glands. 


Axial  lumen. 


Fig.  208. — Sections  of  Tubules,  from  Lingual  Glands,  Illustrating  the  Differences 

BETWEEN  Mucous  and  Serous  Gland  Cells. 

b,  Empty  mucons  cells  ;  c,  mucous  cells  full  of  secretion  ;  d,  lumen  of  the  tubule.     X  240. 

faintly.  Fully  elaborated  mucus,  however,  may  be  colored  intensely  with 
certain  aniline  dyes,  mucicarmine,  and  Delafield's  haematoxylin.  In  cer- 
tain types  of  mucous  cells  the  pale  secretion  area  is  large  in  all  stages  of 
activity.  When  full  of  mucus,  the  nucleus  is  flattened  against  the  base 
of  the  cell,  and  when  empty,  the  nucleus  becomes  more  oval  without  essen- 
tially changing  its  position  (Fig.  208).  This  differs  from  the  t}^e  of  mu- 
cous cell  found  in  the  gastric  epithelium 
in  which  the  secretion  area  varies  consider- 
ably with  the  elaboration  and  discharge 
of  secretion  (Fig.  35,  p.  33). 

Glands  may  consist  entirely  of  serous 
or  of  mucous  cells,  but  frequently  they  in- 
clude cells  of  both  sorts  and  are  called 
mixed  glands.  The  mixed  glands  con- 
tain some  purely  serous  tubules  or  alveoli; 
the  rest  consist  of  both  mucous  and 
serous  cells,  so  arranged  that  the  latter 
appear  more  or  less  crowded  aAvay 
from  the  lumen.  Often  they  form  a 
layer  outside  of  the  mucous  cells  partly  encircling  the  tubule  or  alveolus 
and  constituting  a  crescent  [demilune].  They  are  shown  in  Fig.  216.  The 
serous  cells  of  the  crescent  are  in  connection  with  the  lumen  by  means  of 
secretory  capillaries  (p.  36)  which  branch  over  their  surfaces,  ending 
blindly,  after  passing  between  the  mucous  cells  (Fig.  209).     Sometimes 


Crescent. 

Fig.  209.  —  From 
Submaxillary 
X  320. 


Intercellular 
secretory 
capillary. 


A   Section   of   the 
Gland   of    a    Dog. 


SEROUS    ORAL   GLANDS. 


187 


Intercellular 
secretorj' 
capillaries. 


the  cells  of  the  crescent  are  directly  in  contact  with  the  lumen.  Since  the 
serous  crescents  are  always  associated  intimately  and  somewhat  irregularly 
with  mucous  cells,  they  were  naturally  interpreted  as  a  functional  phase 
of  the  latter.  It  is  probably  true  that  some  crescents  represent  empty 
mucous  cells  which  have  been  crowded  from  the  lumen  by  those  full  of 
secretion.  No  secretory  capillaries  lead  to  such  mucous  crescents,  which 
moreover  are  not  abundant.  Another  sort  of  crescentic  figure  is  made 
by  the  basal  protoplasm  in  mucous  cells  otherwise  full  of  secretion.  Fi- 
nally, in  oblique  sections,  stellate  cells  associated  with  the  basement  mem- 
brane may  resemble  true  crescents. 

The  oral  glands  include  serous 
glands,  mucous  glands,  and  mixed  glands 
to  be  described  in  turn. 

Serous  Glands. 

The  serous  orar^lands  are  the  parotr 
id  glands  and  the  serous  glands  of  the 
tongue  [v.  Ebner's  glands].  The  latter 
are  branched  tubular  glands  limited  to  tlje 
vicinity  olthe  vallate  and  foliate  pa|)illae. 
Generally  they  open  into  the  grooves 
which  bound  these  papillae.  Their  ducts 
are  lined  with  simple  or  with  stratified 
epithelium,  which  is  occasionally  ciliated. 
Their  small  tubules  consist  of  a  delicate 
membrana  propria  or  basement  mem- 
brane, which  surrounds  the  low  columnar 
or  conical  serous  cells.  In  this  simple 
epithelium,  cell  walls  are  lacking.  With  special  stains  and  high 
magnification  an  outer  dark  granular  zone  has  been  distinguished  from  the 
clear  basal  portion  of  the  cell  which  contains  the  nucleus.  The  lumen  of 
the  tubules  is  very  narrow  and  receives  the  still  narrower  intercellular 
secretory  capillaries  (Fig.  210). 

The  parotid  glands  are  the  largest  oral  glands.  Each  is  situated  in 
front  of  the  ear  and  is  folded  around  the  ramus  of  the  mandible;  its  duct, 
th.e_parotid  duct  rStenson's]j_empties  into  the  mouth  opposite  the  second 
molar  tooth  of  the  upper  jaw.  The  parotid  gland  is  an  organic,  branched 
serous  gland,  subdi\ided  into  lobes  and  lobules.  The  accessory  parotid 
gland  appears  as  a  lobe  separated  from  the  others.  The  parotid  duct  is 
characterized  by  a  thick  membrana  propria  and  consists  of  a  Ia^-q  la}"crcd 
columnar  epithelium  with  occasional  goblet  cells.     As  the  duct  branches 


Fig.  210.  —  Section  of  a  Serous  Gland 
FROM  THE  Tongue  of  a  Mouse.   X  240. 

Prepared  by  Golgi's  method,  a  precipitate 
has  formed  in  the  ducts.  The  right 
lower  part  of  the  figure  has  been  com- 
pleted by  adding  the  cell  outlines. 


l88  HISTOLOGY. 

repeatedly,  the  epithelium  becomes  a  simple  cpliimnarjepithelium,  after 
being  pseudostratified,  with  two  rows  of  nuclei  (Fig.  27,  p.  28).  Possibly 
the  epithelium  near  the  outlet  of  the  duct  is  also  pseudostratified.  The 
excretory  portion  of  the  duct  is  followed  by  the  secretory  part  formed  of 
simple  columnar  cells  with  basal  striations,  perhaps  indicative  of  secretory 
activity.  As  showTi  in  the  diagram,  Fig.  211,  and  in  the  sections.  Figs. 
212  and  213,  the  secretory  duct  becomes  slender,  making  the  intercalated 
ducts.  They  are  lined  by  flat  cells,  longer  than  they  are  wide,  and  these 
form  a  continuous  layer  with  the  large  cuboidal  serous  gland  cells  of  the 
terminal  alveoli.     The  gland  cells  w^hen  empty  of  secretion  are  small  and 


■'•«k. 


'^^M^ 


Excretory 
duct. 


End  pieo 


fish's Secretory 

Wl                           duct. 

\                       .  Intercalated 

\                               duct. 

Intercalated 

/  ^^N_.--g-       End  pieces. 

duct. 

f^  ^' 

Fig.  211.— Diagram  of  the 

Fig. 

Hu.MAN  Parotid  Gland. 

Fig.  212. — Section  of  the  Parotid  Gland  of  an 

Adult  Man.     X  252. 

The  very  narrow  lumen  of  the  alveolo-tubular  end  pieces 

is  not  shown. 


darkly  granular,  and  when  full  are  larger  and  clearer.  They  rest  upon  a 
basementjaf;mhT:anp...r.QJltaining  stellate  cells.  Intercellular  secretory  cap- 
illaries end  blindly  before  reaching  the  basement  membrane. 

The_jlyeoli^_of_llie-  parotid  gland  are.  somewhat  elongated,  and  are 
branched.  Between  them  there  is  vascular  connective  tissue  containing 
fat  cells.  In  denser  form  it  surrounds  the  lobules  and  lobes  of  the  gland, 
and  the  larger  ducts.  The  ducts  which  are  found  in  the  connective  tissue 
septa  are  called  interlobular  ducts,  in  distinction  from  those  which  are 
surrounded  by  the  alveoli  in  which  they  and  their  branches  terminate. 
The  latter  are  intralobular  ducts.  They  are  smaller  and  have  less  con- 
nective tissue  around  them  than  the  interlobular  ducts,  of  which  however 


SEROUS   ORAL   GLANDS. 


189 


they  are  the  continuations.  The  arteries  generally  follow  the  ducts  from 
the  connective  tissue  septa  into  the  lobules,  where  they  produce  abundant 
capillary  networks  close  to  the  basement  membranes.  The  veins  derived 
from  these  soon  enter  the  interlobular  tissue  and  may  then  accompany 
the  arteries.  Lymphatic  vessels  also  follow  the  ducts  and  branch  in  the 
interlobular  connective  tissue  where  they  terminate.  Only  tissue  spaces 
have  been  found  within  the  lobules.  The  nerve  supply  requires  further 
investigation.  .Sympathetic  nerves  from  the  plexus  around  the  carotid 
artery  accompany  the  blood  vessels  into  the  parotid,  and  by  controlling 
the  blood  supply  have  an  important  bearing  upon  secretion.  The  great 
auTicuTajTigrV(E7ffo'm  the  second  and  third  cervical  nerves,  enters  the  gland, 
and  branches  of  the  facial  nerve  are  involved  in  it,  but  branches  from  the 
otic  ganglion  are  considered  the  essential  nerves  to  the  gland  cells.  In  the 
other  sahvary  glands  which 


Fat  cell 

Aheolus 
Intercalated  duct  ^  i; 

longitudinal  section   ^aC^ 
cross  section 


H^ 


'^^ 


have  been  more  thoroughly 
studied,  nonmedullated 
fibers  from  the  sympathetic 
ganglia,  either  outside  of 
the  gland  hke  the  otic  or 
from  microscopic  ganglia 
along  its  larger  ducts,  form 
plexuses  beneath  the  base- 
ment membranes.  Fibers 
from  these  plexuses  pene- 
trate the  membranes, 
within  which  they  form 
another  netw^ork  before  ter- 
minating in    contact  with 

the  epitheHal  cells.  Their  endings  may  be  simple  or  branched,  and 
are  varicose.  Free  sensory  endings  of  medullated  fibers  are  said  to  occur 
in  the  epithelium  of  the  ducts. 


-Section  of  the  Parotid  Gland  from  a 
Man  of  23  Years.     X  80. 


Mucous  Glands. 
The  pure  mucous  glands  of  the  mouth  are  simple  branched  alveolo- 
tubular  glands  found  only  on  the  anterior  surface  of  the  soft  palate  and  on 
the  hard  palate  (palatine  glands),  along  the  borders  of  the  tongue  (lingual 
glands),  and  in  greater  numbers  in  the  root  of  the  tongue.  There  they 
may  open  into  the  tonsillar  pits  through  ducts  fined  with  columnar  epi- 
thelium., sometimes  ciHated.  The  wall  of  the  tubules  consists  of  a  struc- 
tureless basement  membrane  and  of  columnar  mucous  cells,  varying  ac- 
cording to  their  functional  condition  as  shown  in  Fig.  208,  I-II.     The 


1 9© 


HISTOLOGY. 


empty  cells  are  smaller  than  the  others,  and  the  nuclei,  though  at  the  base 
of  the  cell  and  transversely  oval,  are  not  as  flat  as  in  cells  full  of  secretion. 
Seldom  can  cells  be  found  completely  occupied  by  unaltered  protoplasm. 
A  single  gland,  or  even  a  single  alveolus,  may  contain  cells  in  different 
phases  of  secretion,  as  is  clearly  seen  when  special  mucin  stains  are  used. 
Secretory  capillaries  are  not  found  in  the  purely  mucous  glands. 

Mixed  Glands. 
The  mixed  oral  glands  are  the  subhngual,  submaxillary,  anterior  lin- 
gual, labial,  buccal,  and  molar  glands.     They  all  possess  crescents  of  serous 
cells  such  as  are  to  be  described  in  the  largest  glands  of  this  group, — the 
sublingual  and  submaxillary. 


Secretory  duct 


■Crescents. 


Mucous 

cells. 


3 


Serous 
cells. 


I^eucocytes. 
•'■'^  Blood  vessel. 


Fig.    214.  —  Diagram     of     thk 
Human  Sublingiai.  Gland. 


Fic;.    215. — Section    of    the  Sublingual   Gl.'^nd    from    .a 
M.-\N  OF  2\  Ve.^rs.     X  80. 


The  sublingual  glands  are  two  groups  of  glands,  one  on  either  side  of 
the  median  line,  under  the  mucous  membrane  in  the  front  of  the  mouth. 
The  largest  component  is  an  alveolo-tubular  structure  emptying  by  the 
ductus  sublingualis  major  on  the  side  of  the  frenulum  linguae.  The  main 
stem  and  the  principal  branches  of  the  large  sublingual  duct  are  lined  by 
a  two-layered  or  pseudostratified  columnar  epithelium,  as  in  the  parotid 
duct.  They  are  surrounded  by  connective  tissue  containing  many  elastic 
fibers.  Ducts  less  than  .05  mm.  in  diameter  have  a  simple  columnar 
epithelium,  which  in  a  few  places  becomes  low  and  basally  striated  to  form 
the  secretory  ducts  (also  called  sahvary  ducts).  As  shown  in  the  dia- 
gram, Fig.  214,  the  secretory  ducts  arc  very  short,  and  narrow  intercalated 
ducts  are  absent.     The  tubules  are  surrounded  by  basement  membranes 


MIXED    ORAL    GLANDS.  igi 

containing  stellate  cells,  and  consist  of  both  serous  and  mucous  cells. 
The  crescents  are  often  very  large  and  include  manv  cells.  Onlv  the 
serous  cells  are  provided  with  the  branched  intercellular  secretorv  capil- 
laries. The  connective  tissue  between  the  tubules  and  lobules  contains 
many  leucocytes.  The  nerves  are  arranged  as  described  for  the  parotid 
gland.  The  gland  cells  are  supphed  by  sympathetic  fibers  from  adjacent 
subhngual  ganghon  cells,  about  which  fibers  from  the  chorda  tympani  may 
arborize.  The  latter  are  said  not  to  proceed  directly  to  the  dand  cells. 
Sensory  nerves  to  the  ducts  may  come  from  the  hngual  branch  of  the  man- 
dibular nerve. 

Besides  the  gland  just  described  there  are  from  8  to  20  small  separate 


A  crescent  consisting  of 
eight  serous  cells. 


Part  of  an  e\cretor\  duct 


Tangential 
section  of 
serous  cells. 


Cross  section 
with  mucous 
cells  and  (left) 
thick  menu 
brana  propria. 


Lumen 


Fig.  216.— Section  of  a  Hu.man  Sublingl'al  Gla.nd.     x  252. 

alveolo-tubular  glands  closely  joined  to  it,  and  described  as  part  of  the  sub- 
lingual gland.  They  open  by  separate  ducts,  the  ductus  siihlinguales 
minores.     They  all  ( ?)  consist  almost  exclusively  of  mucous  cells. 

The  submaxillary  glands  are,  branched  alveolar  glands,  in  part  tubulo- 
alveolar,  found  within  the  lower  border  of  the  mandible,  each  being  drained 
by  a  submaxillary  duct  [Wharton's]  which  opens  on  the  sides  of  the  frenu- 
lum linguae  near  its  front  margin.  Its  orifice  may  be  hned  by  stratified 
epithelium,  but  this  soon  gives  place  to  the  two  layered  form.  Serrpfnry 
4ii£ts3re3^elLde\Tlp^jd_XFjg^^^  their  striated  cells  contain  a  yellow 

^^EL.  ^^^  intercalated  ducts,  which  are  lined  with  simple  cuboidal 
epithelium,  lead  to  terminations  of  two  sorts.     Most  of  these  consist  en- 


192 


HISTOLOGY. 


tirely  of  serous  cells.     The  others  are  mixed,  but  the  crescents  are  small, 
composed  of  only  a  few  or  even  of  single  serous  cells.     Secretory  capillaries 


Excretory 
duct. 


C- 


^\. 


Blood  \  essels. 


-,  Sccietoi  vduct. 


Secretory 
duct. 


;\  Intercalated     liveti-vut 
jf^         ducts. 


ConiRc-  ^-^l        ^, 


.i-^'C'^'^Mlr 


^   Mucous 
^       cells. 
Cres- 


nt. 


Serous  end  piece.        Fat  cell. 


End  pieces. 


Fig.  217.— Di.agr.\m  of  the   Hum.an  Fig.  21S. — Section  of  the  Sibm.axill.arv  Gland  from 

Sl'bm.\xillarv  Gland.  a  Man  of  23  ^'ears.     X  So. 

such  as  have  already  been  described,  are  related  only  to  the  serous  cells. 

Elastic  tissue  surrounding  the  ah-coli  has  been  thought  to  aid  in  expelhng 


Seiou"-  ^Hnd  cells 


Inlcicil-ited  duct. 
/ 

Mucous 
gland  cells. 


Connective  tissue. 


Lumen. 


Crescent. 


Secretory  duct. 


Fig.  219.— Section  of  the  Slb-maxillarv  Gland  of  an  Adilt  Man.     'A  252. 

the  secretion  through  the  ducts.     It  is  known  that  the  secretion  is  ehmi- 
"riated  from  the  gland  cells  under  high  pressure,  and  so   would  not  be 


MIXED    ORAL    GLANDS.  1 93 

checked  by  this  action  of  the  elastic  membranes.  The  nerves  are  sympathetic 
fibers  from  ^the  submaxillary  gangHon  and  microscopic  gangha  along  the 
ducts.  The  chorda  tympani  does  not  send  fibers  directly  to  the  gland  cells. 
Sensory  nerves  may  be  derived  from  the  branches  of  the  mandibular  nerve. 
In  the  oral  glands,  not  infrequently  degenerating  lobules  occur,  char- 
acterized by  abundant  connective  tissue  between  tubules  with  wide  lumens 
and  low  gland  cells.     Sometimes  they  are  surrounded  by  leucocytes. 

The  Development  of  the  Digestive  Tube. 

The  early  development  of  the  entoderm  has  been  described  in  the 
section  on  general  histogenesis  (page  i8).  At  first  it  forms  a  layer  fining 
the  blastodermic  vesicle.  Then  by  a  process  of  folding  and  constriction 
the  'pharynx'  develops  from  its  anterior  part  so  that  the  entire  entoderm 
is  shaped  somewhat  like  a  chemist's  retort.  The  bulbous  expansion  is 
the  fining  of  the  yolk  sac.  An  analogous  stage  has  been  described  in  the 
chick  embryo  (Fig.  20),  where,  in  place  of  a  thin  walled  yolk  sac,  there  is 
a  solid  mass  of  yolk-laden  entoderm.  From  the  posterior  wall  of  the  yolk 
sac  an  entodermal  outpocketing  is  produced,  which  rapidly  becomes  long 
and  slender.  It  is  called  the  allantois  (Fig.  220,  al.).  At  first  the  allan- 
tois  is  directed  posteriorly  but  soon  it  swings  ventrally  and  then,  as  in  C, 
it  passes  from  the  hind  end  of  the  digestive  tract  along  the  ventral  body 
wall  into  the  umbilical  cord.  The  part  within  the  cord  becomes  a  strand 
of  cells.  Within  the  body,  that  portion  of  the  allantois  which  is  toward 
the  umbilicus  or  navel,  becomes  subsequently  a  fibrous  remnant,  the 
urachus,  which  leads  from  the  navel  to  the  bladder.  The  bladder  is  the 
dilated  lower  part  of  the  aUantois,  and  is  therefore  fined  wdth  entoderm, 
being  embryologically  a  part  of  the  digestive  tube. 

In  mammafian  embryos  the  aUantois  and  the  intestinal  tract  connect 
freely  at  their  posterior  ends,  and  the  entodermal  area  common  to  both  is 
called  the  cloaca.  Here  the  entoderm  comes  in  contact  with  the  ectoderm 
and  forms  the  cloacal  membrane,  a  structure  comparable  with  the  oral 
membrane.  After  this  membrane  disappears  there  is  no  apparent  fine 
of  separation  between  the  ectoderm  of  the  skin  and  the  entoderm  of  the 
•cloaca.  In  this  region  in  both  sexes  a  conical  elevation,  the  genital  papilla, 
is  formed,  and  the  cloaca  with  its  lateral  walls  closely  approximated  is 
found  within  it.  Gradually  the  allantois  becomes  divided,  from  the  intes- 
tinal tract  as  shown  in  Fig.  220,  B,  C,  and  D.  The  mesenchymal  tissue 
between  them  thus  comes  in  contact  with  the  ectoderm  to  produce  the 
perineum  which  divides  the  cloaca  into  the  urogenital  sinus  ventrally  and 
the  anus  dorsally.  In  E,  the  bladder  is  seen  to  terminate  in  the  urethra 
which  in  the  male  is  considered  to  be  chiefly  an  elongation  of  the  ecto- 
13 


194 


HISTOLOGY. 


dermal  part  of  the  urogenital  sinus;  only  the  part  toward  the  bladder, 
which  corresponds  with  the  urethra  in  the  female,  is  described  as  ento- 
dermal.  As  already  noted  there  is  no  hne  of  demarcation  between  the 
germ  layers  at  this  point,  and  a  portion  of  the  female  urethra  is  by  some 
considered  ectodermal.  The  bladder  is  to  be  described  with  the  urinary 
organs  and  the  urethra  with  the  genital  organs. 

Returning  to  the  intestinal  portion  of  the  entodermal  tract,  it  is  seen 
that  in  early  stages.  A,  the  yolk  sac  extends  from  the  pharynx  nearly  to  the 


Fig.  220. — Stages  in  the  Development  of  the  Digestive  Tube.     A,  Rabbit  of  q  ciay.s.      B,  Man 

2.15  mm.  (after  His).     C,  Pig,  12  mm.     D,  Man,  17.8  mm.  (after  Thyng).     E,  Man,  about  5  months, 
a.,  Anus;  al.,  allantois  ;  bl..  bladder  ;  cae.,  caecum  ;  cl.,  cloaca  ;  du.,  duodenum  ;  I.  i.,  large  intestine;  oe., 

oesophagus;  p.,  penis;  ne.,  perineum  ;    ph..  pharynx  ;  r.,  rectum  ;  s.  i.,  small  intestine  ;  St.,  stomach  ; 

u.  c.  umbilical  cord  ;    ur..  urethra  ;    ura.,  urachus  ;    u.  S.,  urogenital  sinus  ;    v.  p.,  vermiform  process  ; 

y.  s.,  yolk  sac  ;  y.  St.,   yolk  stalk. 


posterior  limit  of  the  entoderm.  With  further  growth  a  posterior  intestine 
becomes  formed  by  folding  or  constriction,  comparable  with  the  pharynx 
in  front  (B).  The  connection  between  the  yolk  sac  and  the  intestine  be- 
comes a  slender  yolk  stalk,  a  part  of  which  is  shown  in  C  and  D.  Later 
it  loses  its  continuity  and  the  detached  yolk  sac  remains  until  birth  as  a 
small  vesicle  at  the  distal  end  of  the  umbilical  cord,  with  which  it  will  be 
described  later.  The  yolk  stalk  which  extends  from  the  umbilicus  to  the 
intestine  should  be  completely  resorbed.  It  may  persist  as  a  fibrous  cord 
liable  to  produce  intestinal  obstruction,  or  the  part  near  the  intestine  may 


DIGESTIVE    TUBE. 


195 


remain  as  Meckel's  diverticulum.  This  is  a  blind  pouch  of  intestine, 
usually  less  than  four  inches  long  but  sometimes  much  longer,  found  on 
the  small  intestine  some  four  feet  from  its  termination. 

Anterior  to  the  yolk  stalk  the  entodermal  tube  forms  successively  the 
pharynx,  oesophagus,  stomach,  duodenum,  and  the  greater  part  of  the 
small  intestine;  posterior  to  it,  the  remainder  of  the  s^nall  intestine,  the 
large  intestine  and  the  rectum.  The  rectum  terminates  at  the  anus  which 
is  formed  as  an  ectodermal  inpocketing  closed  in  embryonic  hfe  by  the  anal 
membrane.  Rarely  this  membrane  or  the  adjacent  rectum  remains  im- 
perforate at  birth.  A  transient  embryonic  extension  of  the  intestine  be- 
yond the  anus  toward  the  tail  is  known  as  "post-anal  intestine."  It  early 
disappears,  and  has  not  been  drawn  in  Fig.  219.  The  stomach  is  a  dilated 
portion  of  the  tube  at  first  vertically  placed  in  the  median  plane  (C)  but 
later  so  turned  that  its  left  side  is  ventral  (or  anterior),  as  in  D.  The  duo- 
denum is  a  subdivision  of  the  small  intestine,  the  remainder  of  which  is 
arbitrarily  divided  into  the  jejunum  (the  anterior  two  fifths)  and  the  ileum 
(the  posterior  three  fifths).  Where  the  ileum  joins  the  large  intestine  a 
bHnd  outpocketing  of  the  latter  occurs,  consisting  of  the  caecum  and  its 
slender  prolongation  the  vermijorm  process  (processus  vermiformis).  At 
a  certain  stage  (C)  the  intestines  make  a  simple  loop  of  which  the  large 
intestine  forms  the  posterior  or  lower  hmb.  To  produce  the  arrangement 
characteristic  of  the  adult,  the  loop  becomes  twisted,  as  in  D,  so  that  the 
large  intestine  crosses  the  small  intestine  not  far  from  the  stomach;  thus  it 
is  possible  for  the  large  intestine  nearly  to  encircle  the  small  intestine  which 
becomes  greatly  convoluted,  without,  however,  changing  its  fundamental 
relations.  Besides  the  vermiform  process  and  caecum,  the  large  intestine 
includes  the  ascending,  transverse,  descending  and  sigmoid  colon,  the  last 
terminating  at  an  arbitrary  line  at  the  rectum.  The  rectum  proceeds  to 
the  anus,  but  not  straight  as  its  name  implies. 

The  entoderm  forms  only  the  epithelial  lining  of  the  digestive  tube 
and  that  of  its  associated  glands.  (Besides  innumerable  accessory 
glands  these  include  the  Hver,  pancreas,  and  the  lungs.)  Around  the  ento- 
derm, the  mesenchyma  forms  successively  the  following  layers, — the  tunica 
propria  which  contains  the  reticular  tissue  and  lymph  nodules,  and  the 
muscularis  mucosae,  a  thin  layer  of  muscles.  The  epithelium,  tunica  pro- 
pria, and  muscularis  mucosae  together  constitute  the  mucous  membrane. 
It  rests  on  the  tela  suhmucosa,  a  vascular  connective  tissue  layer  containing 
the  sympathetic  plexus  submucosus.  The  submucous  layer  is  followed  by 
the  tunica  muscularis.  This  consists  of  two  or  more  layers  of  muscle  fibers 
between  which  is  the  sympathetic  plexus  myentericus.  Beyond  the  mus- 
cularis is  the  connective  tissue  tu?iica  adventitia  in  case  the  intestinal  tube  is 


196 


HISTOLOGY, 


uncovered  by  peritonaeum,  or  the  tunica  serosa  if  the  peritonaeum  is  present. 
The  following  account  of  the  subdivisions  of  the  digestive  tube  is  essentially 
a  description  of  modifications  in  these  fundamental  layers. 

Oesophagus. 
The  oesophagus  is  a  tube  about  9  inches  long,  the  several  layers  of 
which  are  continuous  anteriorly  with  those  of  the  pharynx,  and  posteriorly 
with  those  of  the  stomach.  It  is  lined  with  a  stratified,  many  layered 
epithelium  like  that  of  the  pharynx.  The  free  surface  which  is  smooth 
but    thrown  into  coarse  longitudinal  folds,   (Fig.   221)  is  covered  with 


Stratified  epithe-  -, 
liuni. 

I     Mucous 
Tunica  propria.     ]  membrane. 
/        yMuscularis  i 
/  y         mucosae. 

/       Submucosa. 


Group  of  fat  c 


■./■■^\    Circular  muscles.)  .. 
■  '       ^  Longitudinal  nnis-     ""SCU- 

cles.  )    'af's- 

^  Tunica  adventitia. 


I 

Lymph  nodule. 


Mucous  gland. 
Fig.  221.— Transverse  Section  ok  the  I'ppkr  Third  of  the  Human  Oesophagus.     X  5- 


squamous  cells;  the  basal  surface  is  indented  by  papillae  of  the  tunica  pro- 
pria. A  muscularis  mucosae,  consisting  of  longitudinal  smooth  muscle 
fibers,  arises  at  the  level  of  the  cricoid  cartilage  and  continues  into  the 
stomach.  At  its  anterior  end  it  begins  as  scattered  bundles  inside  the 
elastic  layer  of  the  pharynx,  and  as  the  muscles  increase  to  form  a  distinct 
layer,  the  elastic  lamina  terminates.  Beneath  the  muscularis  mucosae 
is  the  submucosa,  containing  the  bodies  of  the  oesophageal  mucous  glands. 
They  are  tubulo-alveolar  branched  glands,  with  bodies  about  2  mm.  long, 
and  closely  resemble  those  of  the  mouth.  Crescents  and  serous  cells  are 
absent,  although  empty  cells  may  suggest  the  latter.     Their  ducts  pass 


OESOPHAGUS. 


197 


':^ 


spirally  through  the  muscularis  mucosae  and  tunica  propria,  entering  the 
epithelium  where  it  projects  outward  between  the  connective  tissue  papillae. 
The  ducts  generally  slant  toward  the  stomach.  The  large  ones  are  lined 
with  stratified  epithehum,  often  ciliated,  and  sometimes  they  present  cyst- 
hke  dilatations.  The  smaller  ducts  are  of  simple  epithehum.  Lymphocytes 
may  be  numerous  along  the  ducts,  forming  solitary  nodules  near  them  in  the 
tunica  propria,  and  extending  into  the  submucosa.  Sometimes  the  glands 
show  signs  of  degenera- 
tion. Their  number 
varies  greatly  in  diii'erent 
individuals.  Usually 
they  are  most  abundant 
in  the  upper  half  of  the 
oesophagus. 

A  second  type  of 
oesophageal  glands 
closely  resembles  the  car- 
diac glands  found  in  the 
oesophageal  end  of  the 
stomach.  The  oesopha- 
geal cardiac  glands  (Fig. 
222)  occur  in  the  poste- 
rior or  lowest  2  to  4  mm. 
of  the  oesophagus,  and 
also  in  small  numbers 
at  its  anterior  end  be- 
tween the  levels  of  the 
cricoid  cartilage  and  the 
fifth  tracheal  ring.  The 
latter  group  is  said  to 
be  absent  in  about  30% 
of  the  cases  examined. 
The  bodies  of  the  oesoph- 
ageal cardiac  glands  are  confined  to  the  tunica  propria,  and  their 
ducts  enter  the  epithehum  at  the  summit  of  a  connective  tissue 
papilla.  Their  ducts  have  many  branches,  hned  throughout  \Adth  simple 
columnar  epithelium,  and  this  form  of  epithelium  may  spread  around  their 
outlets  in  the  lumen  of  the  oesophagus.  Because  of  this,  when  the  oesoph- 
agus is  opened,  the  anterior  cardiac  glands  may  appear  macroscopically 
on  its  lateral  walls  as  small  erosions  of  the  fining.  The  secreting  cells  of  the 
cardiac  glands  contain  round  nuclei  and  granular  protoplasm.     Although 


FiG.  222. — Longitudinal  Section  through  the  Junction  of 
THE  Oesophagus  and  Stomach  of  Man.  X  121.  (Scliaf- 
fer,  from  Bailey's  Histology.) 

Oe.,  Oesophagus,  its  stratified  epithelium,  E.,  terminating  at  u  : 
M,  stomach  ;  cd,  dd,  cardiac  glands  in  stomach  and  oesophagus 
respectively;  ac,  wd,  dilated  ducts  of  the  cardiac  glands;  S, 
tunica  propria;  m.  iti.,  muscularis  mucosae. 


198  HISTOLOGY. 

they  are  not  generally  considered  mucous  cells,  it  has  been  found  that  in  the 
stomach  their  protoplasm  responds  to  concentrated  mucin  stains,  and  it  is 
quite  possible  that  they  produce  a  variety  of  mucin.  Occasionally  the 
oesophageal  cardiac  glands  possess  a  few  parietal  cells  hke  those  found  in 
the  stomach.  Cystic  enlargements  and  dilated  ducts  occur,  as  sho^^^l  in 
Fig.  222.     No  special  function  has  been  assigned  to  the  cardiac  glands. 

Beneath  the  submucosa  is  the  tunica  muscularis,  consisting  of  an 
inner  layer  of  circular  or  obhque  fibers,  and  an  outer  layer  of  longitudinal 
fibers.  In  the  anterior  or  upper  part  of  the  oesophagus  the  longitudinal 
fibers  predominate.  The  muscles  there  are  chiefly  striated  and  are  con- 
tinuous with  those  of  the  pharynx.  Gradually  they  are  replaced  by 
smooth  fibers  so  that  the  striated  forms  are  infrequent  in  the  lower  half  of 
the  oesophagus.  At  its  lower  end  the  circular  fiber  layer  is  said  to  be  three 
times  as  thick  as  the  longitudinal.  The  oesophageal  muscles  are  joined  by 
slips  from  the  trachea,  left  bronchus,  aorta,  and  other  adjacent  structures. 

Outside  of  the  muscularis  is  the  connective  tissue  adventitia.  It  con- 
tains branches  of  the  sympathetic  nerves  and  the  oesophageal  plexus  of  the 
vagus  nerv^es.  From  these,  the  nerves  invade  the  muscularis  forming 
the  ganghonated  myenteric  plexus  between  its  layers,  and  pass  on  into 
the  submucosa  where  they  constitute  a  poorly  developed  submucous  plexus. 
The  terminal  branches  include  free  sensory  endings  in  the  stratified  epi- 
thehum,  motor  plates  on  the  striated  muscles  and  the  simpler  motor  end- 
ings on  the  smooth  muscle.  The  blood  vessels  form  capillary  networks  with 
meshes  between  and  parallel  with  the  muscle  fibers.  They  also  branch 
irregularly  in  the  submucosa,  and  form  terminal  loops  in  the  papillae 
of  the  tunica  propria.     Lymphatic  vessels  are  numerous. 

Stomach. 

The  inner  surface  of  the  stomach  presents  macroscopic  longitudinal 
folds  which  become  coarse  and  prominent  as  the  organ  contracts.  There 
are  also  polygonal  areas  from  i  to  4.5  mm.  in  extent,  bounded  by  shallow 
depressions  under  which  the  gastric  glands  have  been  said  to  be  fewer  and 
shorter  than  elsewhere.  The  depressions  are  also  ascribed  to  the  contrac- 
tion of  the  muscles  in  the  mucous  membrane.  Toward  the  pylorus,  or 
duodenal  end  of  the  stomach,  there  are  small  leaf-hke  elevations  of  the 
mucous  membrane,  called  plicae  villosae.  They  may  connect  with  one 
another  to  form  a  network.  The  gastric  mucosa  is  pinkish  gray  since  its 
epithehum  is  thin  enough  to  transmit  the  color  of  the  blood  beneath;  this 
is  not  true  of  the  oesophagus,  the  lining  of  which  appears  white. 

The  epithelium  of  the  stomach  is  simple  and  columnar,  the  transition 
from  the  stratified  epithelium  of  the  oesophagus  being  abrupt  (Fig.  222). 


STOMACH. 


199 


Its  cells  produce  mucus  and  may  be  divided  into  a  basal  protoplasmic  por- 
tion containing  the  elongated,  round,  or  sometimes  even  flattened  nucleus; 


Epithelium. 


^?v^ 


Tunica  propria.   —^ 


0  /i 


^< 


o    6^ 

0      1 


>  Gastric  pit. 


Parietal  cells. 


Chief  cells. 


^i 


Leucocytes. §'^   ^*     »     M  '^'    ,        °( 


Neck. 


Body. 


Smooth  muscle  fibers. 


Parietal  cell. 


Gastric  gland. 


Fundus. 


Fig.  223.— Vertical  Secfion  of  the  Mucous  Membra.ne  of  a  Human  Stomach,  showing 
Gastric  Glands  (Glandulae  Gastricae  Propriae).     X  220. 

and  an  outer  portion  containing  the  centrosome  and  the  secretion  area. 
The  area  varies  in  size,  sometimes  being  large  enough  to  suggest  goblet 


200 


HISTOLOGY. 


Portion  of  a  parietal  cell. 


Chief  cell. 


Parietel  cell. 


'^'* 


vSy 


Gland  lumen. 


Fig.  224. — Transverse  Section  of  a 
Human  Gastric  Gland.    X  240. 


Axial  lumen. 


Parietal  cells  with 
intracellular  se- 
cretory    capillar- 


cells.  It  may  cause  the  free  surface  of  the  cell  to  bulge,  and  in  preserved 
tissue  to  rupture,  but  this  may  be  due  to  reagents.  The  mucus  of  the  gas- 
tric cells  responds  less  readily  to  mucin  stains  than  that  of  the  intestinal 

goblet  cells.  It  first  appears  in  granular 
form.  The  gastric  epithehum  Hnes  a 
great  many  closely  adjacent  gastric  pits 
(foveolae)  into  the  bottom  of  which  the 
glands  of  the  stomach  empty.  These 
glands  are  of  three  sorts,  the  gastric 
glands,  cardiac  glands,  and  pyloric  glands. 
None  of  them  extend  through  the  mus- 
cularis  mucosae  into  the  submucosa.  The  cardiac  glands  are  limited  to 
the  oesophageal  end  of  the  stomach,  occupying  a  zone  from  5  to  40  mm. 
wide;  the  pyloric  glands  may  extend  from  6  to  14  cms.  from  its  duodenal 
end;  and  the  gastric  glands  occur 
throughout  its  body  and  fundus. 

The  gastric  glands  [fundus  glands, 
peptic  glands]  are  straight  or  somewhat 
tortuous  tubular  glands  with  narrow 
lumens,  several  of  which  empty  into  a 
single  gastric  pit  (Fig.  223).  The  pits 
are  sometimes  considered  to  be  the 
ducts  of  the  glands.  The  tubules 
may  join  one  another  before  entering  a 
pit,  so  that  they  may  be  described  as 
branched.  They  are  somewhat  nar- 
rowed toward  the  pits,  forming  the 
neck  of  the  glands;  their  slightly  ex- 
panded base  is  called  the  fundus.  Each 
tubule  consists  of  cells  of  two  sorts, 
chief  cells,  and  parietal  cells.  The 
chief  cells  in  fresh  tissue  appear  dark 
and  filled  with  refractive  granules;  in 
stained  specimens  they  are  clear,  cu- 
boidal  or  low  columnar  structures  en- 
closing round  nuclei.  After  death  the 
chief  cells  rapidly  disintegrate.  Their 
granules,  which  are  often  destroyed  by 
reagents,  are  coarse  toward  the  lumen 

and  fine  in  the  basal  protoplasm.  In  the  absence  of  food  the  chief  cells 
enlarge  and  the  granules  accumulate,  but  with  prolonged  activity  the  cells 


Intercellular  secre 
tory  capillaries. 


Chief  cells 


Fig.  225. — Golgi  Preparation,  showing  the 
Secretory  Capillaries  in  Gastric 
Glands.     X  230. 


STOMACH. 


20I 


become  small;  granules  disappear.  They  do  not  respond  to  mucin  stains. 
It  is^'supposed  that  the  granules,  called  zymogen  granules,  become  converted 
into  pepsin.     The  chief  cells  form  the  greater  part  of  the  gastric  glands, 


Gastric  pits. 


<^^Ca 


/#> 


■Simple  epithelium 
cut  obliquely,  so 
that  it  appears  to 
be  stratified. 


Tunica  propria. 


Pyloric  gland. 


Sections  of  pyloric 
glands. 


Solitary  nodule. 


^X^.. 


'  '-"^  ? 


Muscularis 
mucosae. 


Fig.  226. — VERTIC.A.L  Section  of  Human  Pyloric  Glands.    X  9°- 

parietal  cells  being  irregularly  distributed  among  them  as  in  Fig.  223.  The 
latter^  are  fewest  toward  the  base  of  the  gland.  Like  the  cells  of  serous 
crescents,  they  appear  crowded  away  from  the  lumen  with  which  they  are 
of  tern  connected  only  by  intercellular  secretory  capillaries  (Fig.  224).     The 


202 


HISTOLOGY. 


ite^'^. 


Mucosa. 


capillaries  form  a  basket-like  network  within  the  protoplasm  of  the  parietal 
cells,  as  may  be  demonstrated  by  the  Golgi  method.  This  produces  a 
black  precipitate  wherever  secretion  is  encountered  TFig.  225).  Short 
intercellular  secretor}-  capillaries  are  found  between  but  not  inside  the  chief 
cells.  In  fresh  preparations  parietal  cells  are  clearer  than  chief  cells.  They 
do  not  disintegrate  so  readily.  In  preser^^ed  specimens  they  appear  as 
large  cells  with  granular  protoplasm  which  stains  deeply  with  aniHne 
dyes,  each  cell  containing  one  or  two  rather  large,  round  nuclei.  After 
fasting,  the  parietal  cells  are  smaller  and  their    intracellular    capillaries 

disappear.  Following 
abundant  meals  they 
enlarge  and  may  con- 
tain vacuoles  due  to 
the  rapid  formation 
of  secretion.  They  are 
thought  to  produce 
hydrochloric  acid,  but 
this  is  not  beyond 
question. 

The  cardiac 
glands  (Fig.  221)  are 
much  branched  tubu- 
lo-alveolar  mucous 
glands,  often  cystic, 
containing  a  few  chief 
and  parietal  cells  in 
tubules.  Those  fur- 
thest from  the  oesoph- 
agus are  the  least 
branched  and  resem- 
ble gastric  glands. 
The  secreting  cells  of  the  cardiac  glands  suggest  those  in  the  necks  of  the 
gastric  glands;  their  mucous  nature  is  not  apparent  and  has  been  but  re- 
cently determined.  Although  cardiac  glands  are  developed  in  many  animals 
much  more  extensively  than  in  man,  nothing  is  known  of  their  special 
function. 

The  pyloric  glands  (Fig.  226)  consist  of  very  deep  pits  and  of  short 
winding  branched  tubules.  Gastric  glands  may  be  mingled  with  them. 
The  pyloric  gland  cells  are  chiefly  mucous,  but  occasional  parietal  cells 
are  found  among  them,  and  in  animals  there  are  dark  thin  cells  apparently 
produced  by  compression.     The  usual  t}'pe  is  columnar  with  a  rounded 


Epithelium. 


Tunica  propria. 


Muscularis 
mucosae. 


Submucosa. 


Inner  circular  layer 
of  muscle. 


Connective  tissue. 


Outer    longitudinal 
laverof  muscle. 


Serosa. 

Fig.  227. — Transversk  Sectio.n  of  the  Wall  of  a 
Human  Stomach. 
The  tunica  propria  contains  glands  standing  so  close  together  that 
its  tissue  is  visible  only  at  the  base  of  the  glands  toward  the  mus- 
cularis mucosae. 


i^Auscularis. 


STOMACH.  -  203 

nucleus  near  its  base,  and  protoplasm  resembling  that  of  chief  cells.  In 
structure  the  pyloric  glands  are  like  the  duodenal  glands,  but  the  latter 
extend  into  the  submucosa. 

The  gastric  glands  are  so  closely  packed  that  but  little  reticular  and 
connective  tissue  of  the  tunica  propria  is  found  between  them  (Fig.  227). 
It  is  sufficient  to  support  the  numerous  capillaries  branching  about  the 
glands,  the  terminal  lymphatic  vessels  and  nen-es,  numerous  wandering 
ceUs  and  a  few  vertical  smooth  muscle  fibers  prolonged  from  the  muscu- 
laris  mucosae  (Fig.  223).  The  lymphatic  vessels  begin  blindly  near  the 
superficial  epithehum  and  pass  between  the  glands  into  the  submucosa 
where  they  spread  out  and  are  easily  seen;  they  continue  across  the  mus- 
cularis  and  pass  through  the  mesenter}'  to  join  the  large  hmphatic 
trunks.  Sohtary  nodules  occur  in  the  gastric  mucosa,  especially  in  the 
cardiac  and  pyloric  regions;  they  may  extend  through  the  muscularis  mu- 
cosae into  the  submucosa.  The  muscularis  mucosae  may  be  divided  into 
two  or  three  layers  of  fibers  ha\ang  different  directions.  The  submacosa 
contains  its  plexus  of  nerves  and  many  vessels,  together  with  groups  of  fat 
cells. 

The  muscularis  consists  of  a  thick  inner  circular  and  a  thin  outer 
longitudinal  layer,  together  v-dth  obhque  fibers  sometimes  described  as  a 
third  and  innermost  layer.  O'^dng  to  the  distension  and  twisting  in  the 
development  of  the  stomach  the  course  of  the  fibers  is  disturbed,  and  in 
small  sections  they  may  appear  to  run  in  every  possible  direction.  The 
two  layers  are  clearly  marked  at  the  pylorus,  where  a  great  thickening  of 
circular  fibers  produces  the  sphincter  muscle.  Longitudinal  fibers  have 
been  said  to  be  involved  in  it  so  that  they  can  act  as  a  chlator  of  the  pylorus. 

The  serosa  consists  of  connective  tissue  with  well  developed  elastic 
nets,  and  of  the  peritonaeal  mesotheHum  interrupted  only  at  the  mesenteric 
attachments.  The  serosa  contains  the  vessels  and  nen'es  which  supply 
the  stomach.  The  nenxs  are  partly  vagus  branches  (the  left  vagus  suppHes 
the  ventral  surface  and  the  right  vagus  the  dorsal  surface  owing  to  the 
rotation  of  the  stomach  during  its  development)  and  partly  s\Tnpathetic 
nerves  from  the  cardiac  plexus.  The  distribution  of  vessels  and  nerves  is 
similar  to  that  in  the  intestine,  which  wiU  be  described  in  detail. 

Small  Ixtesten'e — Duodexum. 
The  mucous  membrane  of  both  the  small  and  the  large  intestine  con- 
tains many  simple  tubular  glands,  which  reach  but  do  not  penetrate  the 
muscularis  mucosae.  They  are  called  intestinal  glands  [crypts  of  Lieber- 
kiihn].  Besides  these,  but  in  the  small  intestine  only,  there  are  cylindrical, 
club-shaped  or  foHate  elevations  of  the  epithehum  and  tunica  propria, 


204 


HISTOLOGY. 


called  villi.  Since  the  villi  are  from  0.2  to  i.o  mm.  in  height  they  may  be 
seen  macroscopically  under  favorable  conditions.  In  Fig.  228,  A,  which 
represents  an  enlarged  surface  view  of  the  hardened  mucosa,  the  orifices 
of  the  intestinal  glands  and  the  projecting  intestinal  villi  are  clearly  indi- 
cated. The  vilU  of  the  duodenum  are  low  (0.2-0.5  mm-)  and  leaf-hke  as 
seen  in  the  reconstruction  Fig.  228,  B. 


Fig.  228. 

A,  Surface  view  of  the  hardened  mucosa  of  the  small  intestine  (after  Koelliker).     B,  Side  view  of  a  wax 
reconstruction  of  the  epithelium  in  the  human  duodenum  (Huber).     i.  g.,  Intestinal  gland  ;  v.,  villus. 

There  is  no  sharper  hne  of  separation  between  the  stomach  and  duode- 
num than  the  sphincter  muscle  of  the  pylorus.  Intestinal  glands  have  been 
recorded  in  the  stomach,  and  pyloric  glands  are  said  to  extend  into  the 
duodenum.  Moreover  the  leaf-hke  duodenal  vilh  resemble  the  villous 
folds  of  the  pylorus. 


Intesti 

nal 

giant 

is.-- 

Epitheli 

ium. 

v 

ijli. 

\ 
Tunica  propria.        ,j 

\ 

1 

'\ii 

\% 

.luscularis  mucosae 
Submucosa. 

.--1 

^ 

m 

Stratum  of  circulai 

muscle. 

Stratum  of  lonj<i- 

tudinal  muscle. 

■ ^ 

.,^, 

Connective  tissue. 

— ' 

Duodenal  gland  in  the 

tunica  propria. 
Plica 
circularis.      Duodenal   glands   in  i /, 

Fat.  the  submucosa.       I  Jvn.^uv.^,>v 


<LlM^ 


Fig.  229.— Longitudinal  Section  of  the  Hum.\n  Duodenum.    X  16. 

The  duodenum  differs  from  the  remainder  of  the  small  intestine  by 
containing  duodenal  glands  [glands  of  Brunner].  These  are  branched  tu- 
bulo-alveolar  structures  which  extend  into  the  submucosa  (Fig.  229). 
To  a  small  extent  they  branch  among  the  intestinal  glands  inside  themuscu- 
laris  mucosae,  as  seen  in  Fig.  230.     Their  ducts  may  either  enter  the  bases 


DUODENUM. 


20: 


Intestinal  glands. 


of  the  intestinal  glands,  or  may  pass  between  them  to  the  surface.  In  form 
and  position  the  duodenal  glands  suggest  those  of  the  oesophagus,  but  in 
structure  they  so  resemble  the  pyloric  glands  as  to  have  been  considered 
identical  with  them.  They  produce  a  mucus  which  stains  with  difficulty, 
and  are  free  from  goblet  cells.  As  in  the  pyloric  glands,  occasional  parietal 
cells  have  been  observed,  found  chiefly  inside  of  the  muscularis  mucosae. 
The  dark  cells  due  perhaps  to  compression,  occur,  and  there  are  intercellular 
secretory  capillaries.  A_structureLess  basement  membrane  surrounds  the 
tubules.  The  duodenal  glands  are 
so  numerous  toward  the  stomach  that 
the  submucosa  may  be  filled  with  their 
tubules.  Th.ey  are  also  abundant  near 
the  duodenal  papilla  where  the  bile  and 
pancreatic  ducts  enter  the  descending 
portion  of  the  duodenum.  Beyond  this 
point  they  become  fewer,  and  disappear 
before  the  end  of  the  duodenum  is 
reached.  Except  for  these  glands  the 
duodenum  is  essentially  like  the  remain- 
der of  the  small  intestine,  described  in 
the  following  section. 


Oblique 
section. 


Transverse 
section. 


Longitudinal 
section. 


Oblique 
section 


Transverse 
section 


Longitudina 

section 


of  the  tubules  of  a  duodenal  gland. 

Fig.  230. — From  a  Section  of  a  Human 

Duodenum.     X  240. 

Only  the  lower  half  of  the  mucosa  and  upper 
half  of  the  submucQsa  are  sketched.  A 
large  portion  of  the  duodenal  gland  lies 
above  the  muscularis  mucosae. 


Small  Intestine — Jejunum  and 
Ileum. 
As  already  stated,  the  small  intes- 
tine is  characterized  by  its  glands  and 
the  villi  which  impart  a  velvety  appear- 
ance to  its  surface.  In  the  jejunum 
the  club-shaped  or  cyHndrical  vilh  are 
more  slender  and  numerous  than  in  the 
ileum;  in  the  distal  portion  of  the  latter 
they  are  short  and  scattered,  finally  dis- 
appearing on  the  colic  surface  of  the  valve  of  the  colon  [ileo- 
caecal  valve].  Each  villus  consists  of  an  epithehal  covering  and  a  core 
of  connective  tissue,  the  tunica  propria  (Fig.  231).  There  are  other  and 
larger  elevations  in  the  lining  of  the  small  intestine,  known  as  circular  folds 
{plicae  circular es)  [valvulae  conniventes].  As  shown  in  Fig.  232,  their 
interior  is  formed  by  the  submucosa  and  their  surface  is  covered  by  the 
entire  mucous  membrane, — viUi,  glands,  and  the  muscularis  mucosae. 
Since  the  tunica  muscularis  does  not  enter  them  they  cannot  be  obHterated 
by  distending  the  intestine.     The  circular  folds  begin  in  the    duodenum 


2o6 


HISTOLOGY. 


(Fig.  229),  and  beyond  the  duodenal  papilla  they  are  talland  close  together. 
They  are  also  highly  developed  in  most  of  the  jejunum,  but  distally,  as  in 
the  ileum,  they  are  lower  and  further  apart.  From  the  last  two  feet 
of  the  ileum;  they  may  be  absent.  As  their  name  imphes,  they  generally 
tend  to  encircle  the  intestine.  They  may  form  short  spirals,  or  branch  and 
connect  with  one  another.  Some  of  them  are  so  oblique  as  to  appear  cut 
transversely  in  cross  sections  of  the  intestine. 


Epithelium. 


Sections  of 
villi. 
I  a 


m^ 


w^ 


Wi 


^f%„ 


i-^'"... 


% 


// 


Tunica, 
proinia. 


Tunica 
propria." 


':\^-'S% 


Muscularis 
mucosae. 


Subinucosa.     Intestinal  glands.  Oblique  sections  of  intestinal  glands. 

P"iG.  231.— Vertical  Section  ok  the  Mucous  Membrane  of  the  Jejunum  of  Adult  Man.     X  80. 

The  space,  a,  betweeji  the  tunica  propria  and  the  epithelium  of  the  villus  is  perhaps  the  result  of  the 
shrinking  action  of  the  fixing  fluid.  At  b  tlie  epithelium  has  been  artificially  ruptured.  The  goblet 
cells  have  been  drawn  on  one  side  of  the  villus  on  the  right. 


There  is^jaoily  .an  arbitrary  separation  between  the  jejunum  and  the 
ileum;  the  latter  contains  fewer  and  shorter  villi,  and  its  circular  folds  are 
more  widely  separated^ 

The  entodermal  epitheliumof  the  small  intestine  is  of  the  simple 
colurnliar  form  and  contains  many  goblet  cells.  Since  that  portion  which 
covers  the  vilh  contains  perhaps  as  many  goblet  cells  as  the  part  which  hnes 
the  glands,  it  has  been  suggested  that  the  latter  are  more  properly  termed 
pits.  At  the  base  nf  the  ulanrjs,  however,  there  are  often  some  cells  con- 
taining coarse  granules,  Jp^i£tiii}.LQ^"  a-^^special  secretion.     Its  nature  has 


SMALL    IXTESTIXE. 


207 


not  been  determined.  __  Such  ceUs;_kno\Mi  as__cells  of  Paneth,  are  invariably 
present  m  the  ileum,  and  often  in  the  jejunum;  they  are  not  found  in  the 
glands  of  the  duodenum,  or  in  those  of  the  large  intestine,  with  the  possible 
exception  of  the  vermiform  process.     They  are  shown  in  Fig.  233. 

The  sides  of  the  glands  are  formed  of  columnar  cells  and  goblet  cells, 
so  arranged  that  the  latter  are  seldom  in  contact  with  one  another.  It  is 
thought  that  any  of  the  cells  may  elaborate  mucus  and  become  goblet  cells, 
in  the  manner  described  and  figured  on  page  2,3-     Mitotic  figures  are  often 


Villi. 


Epithe- 
ium. 


Circular  muscle.  ^ 

Longitudinal    - 

muscle.         

Serosa.  feS--"~ 


Fig.  232.— Vertical  Longitudin.al  Section  of  the  Jejunum  of  Adult  M.a.n.  X  i6. 
The  plica  circularis  on  the  right  supports  two  small  solitary  nodules,  which  do  not  extend  into  the  sub- 
mucosa;  one  of  them  exhibits  a  germinal  center,  X.  The  epithelium  is  slightly  loosened  from  the 
connective  tissue  core  of  many  of  the  villi,  so  that  a  clear  space,  XX,  exists  between  the  two.  The 
isolated  bodies  lying  near  the  villi  (more  numerous  to  the  left  of  the  plicae  circulares)  are  partial 
sections  of  villi  that  were  bent,  therefore  not  cut  through  their  entire  length. 


observed  in  the  glands  and  seldom  elsewhere,     fla  the  stomach  they  occur 

near  the  necks  of  the  glands.)  From  this  it  is  inferred  that  the  outer  cells, 
including  those  of  the  viUi,  are  replaced  from  below,  and  that  the  cells 
toward  the  fundus  of  the  glands  are  renewed  from  above. 

The  epithelial  cells  of  the  villi  are  taller  than  in  the  glands,  and  the 
goblet  cells  are  somewhat  larger.  The_cQlunmar  cells .  are  covered  by  a 
vertically  striated  top  plate  or  cuticula,  which  is  thinner  in  the  outer  part  of 
the^glands  and  is  absent  from^  their  deeper  parts.     The  striation  is  con- 


208 


^<2J^nrx^^ 


^-Cou*.     tp         tto- 


HISTOLOGY. 


Goblet-cells. 


sidered  due  to  protoplasmic  processes  lodged  in  pores.     Terminal  bars  are 
also  present.     The  goblet  cells  have  a  thin  top  plate,  which  in  sections  is 

often  ruptured  to  allow  the 
escape  of  mucus.  This  is 
probably  not  artificial.  Be- 
tween the  epithehal  cells 
there  are  narrow  spaces  into 
which  lymphocytes  often  mi- 
grate (Fig.  234),  and  from 
which  some  of  them  may 
escape  into  the  lumen  of  the 
intestine.  The  lateral  walls 
of  the  epithelial  cells  are  de- 
scribed as  modified  ectoplasm 
rather  than  true  membranes. 
Their  basal  ends  rest  upon 
the  tunica  propria,  which  is 
a  reticular  tissue  containing 
many  small  round  cells  in  its 
meshes  and  supporting  a  cen- 
tral lymphatic  vessel  together 
with  numerous  blood  capil- 
laries (Fig.  235).  Smooth 
from  the  muscularis  mucosae,  and  by 
villus  and  empty  its  l^Tnphatic  vessd. 
and  phagocytes  may  also  be  found  in  the 


Cells  of  Paneth. 

Fig.  233. — Three  Intestinal  Glands  from  Sections  of 
THE  Ileu.m,  the  One  on  the  Right  from  a  Mouse. 
THE  Other  Two  from  a  Man.     X  390.  ■ 


The  left 
fluid, 
and  formaline  mixture 


and  was  from  a  preparation  fixed   in   Zenker's 
fluid,  the  other  two  were  fixed  in  a  potassium  bichromate 


pitlicliuin. 


muscle   libers    extend    into    it 
contraction   they    shorten  the 
Eosinophihc  cells,  plasma  cells 
tunica  propria  of  the  villi. 

Interest  in  the  villi  centers 
chiefly  in  their  relation  to  the 
absorption  of  nutritive  material 
from    the    intestinal    contents 

{chyme) .       Fatj_£robably i^n 

combination,  is  said  to  be  re- 
ceived by  osmosis  through  the 
cuticula.  ■  It  appears  in  vacu- 
oles in  the  outer  part  of  the 
cells,  as  shown  in  osmic  acid 
preparations,  but  neither  within 

nor  in  contact  with  the  cuticula.  It  extends  to  the  deeper  part  of  the 
cells  and  is  found  in  the  intercellular  spaces  between  the  epithehal  cells. 
It  has  been  said  that  lymphocytes  ingest  it  there  and  convey  it  to  the  cen- 


Fic.  234.— Fro.m  a  Section  of  the  S.mall  Intestine 

i-ROM  a  Kitten  Seven  Days  Old.     x  250. 
The   epithelium   on   the   left   contains   many  wandering 

leucocytes   (lymi)hocytes).     The  epithelium   on    the 

risflit  contains  but  three. 


SMALL   IXTESTINE. 


209 


tral  lymphatic,  within  which  they  break  do\Mi  and  set  free  the  fat,  but 
this   explanation   of    the    transfer   is   not   beyond   question.     It   is   weW 


Epithelium. 


# 


V-       \    r^ 


Tunica  propria. 


Portion  of  a  capillary 

blood  vessel. 


Cuticula. 


Xucleus  of    a   lympho- 
cyte. 


Tangential  section  of  a 
goblet  cell. 


Mucus  in  a  goblet  cell. 


NucleuTbf  a  smooth  muscle  fiber.        Central  lymphatic  vessel. 
Fig.  235. — Longitudinal  Section  through  the  Apex  of  the  Villus  of  a  Dog.    X  360. 
The  goblet  cells  contain  less  mucus  as  they  approach  the  summit  of  the  villus. 

kno%^Ti  that  fat  enters  the  lymphatic  vessels  so  that  they  become  distended 
and  ivhite,  their  fatty  contents   being   designated  chyle. 

In  regard  to  the  absorption 
of  proteid  material,  the  observa- 
tions of  Pio  Mingazzini,  which 
have  been  confirmed  by  some 
and  denied  by  others,  are  of  con- 
siderable interest.  As  sho^^^l  in 
Fig.  236,  he  found  that  the  basal 
protoplasm  of  the  epithehum 
presented  an  ordinary  appear- 
ance before  digestion  (A),  but 
that  after  absorption  had  pro- 
gressed, hyaHne  spherules  ap- 
appeared  in  it  (B).  As  these  became  numerous  they  were  detached  from 
the  cells,  forming  a  reticular  mass  between  them  and  the  tunica  propria  (C). 
14 


D 


A 


B 


C 


Fig.  236. — Stages  of  Intestin.al  Absorption  as 
Seen  in  Epithelial  Cells  of  Villi  from  a 
Hen.     (After  Mingazzini.) 

A  and  D,  The  states  of  repose  preceding  and  following 
the  process,     s.,  Spherules. 


iSQlO 


HISTOLOGY. 


After  the  spherules  had  broken  down  and  probably  been  transferred  to  the 
blood  vessels,  the  tunica  propria  entered  into  its  usual  relation  with  the 
shortened  epithehum  (D).  The  basal  protoplasm  was  then  restored. 
Thus  proteid  absorption  was  accomplished  as  a  secretory  process  of  the 
epithehum,  the  product  being  eliminated  from  its  basal  portion.  The 
spherules  accumulate  at  and  near  the  tips  of  the  vilh  in  spaces  which  many 
authorities,  including  Professor  Stohr,  describe  as  due  to  the  artificial 
retraction  of  the  tunica  propria  (Fig.  231,  a).  The  spherules  have  been 
considered  a  coagulum  of  the  fluid  squeezed  from  the  reticular  tissue. 
In  part  they  may  be  the  boundaries  of  the  basal  ends  of  epithelial  cells  on 
the  distal  wall  of  the  villus.     Often  a  delicate  connective  tissue  artificially 


cells. 


Fig.  237.  —  Diagram  of  a 
Mesentery  as  Seen  in 
Cross  Section  of  the 
Abdomen.    (After  Miiiot.) 

a.,  Aorta;  c.  p.,  cavity  of  the 
jieritonaeum ;  int., intestine; 
mes.,  mesentery;  p.  m.  and 
V.  m.,  parietal  and  visceral 
lavers  of  mesothelium. 


Fig.  23S.— Surface  View  of  the  Greater  Omentum  from 
A  Rabbit.    X  240. 
Thick  and  thin  coimective  tissue  bundles  form  meshes.     The  wavy 
striation   of    the   binidles   is   obscured   since    the    preparation   is 
mounted  in  balsam.     At  X  the  epithelial  cells  of  the  opposite  sur- 
face are  visible. 


shrinks  from  an  epithelium,  as  seen  in  Fig.  22,  p.  23.  On  the  other  hand, 
these  considerations  are  familiar  to  those  who  interpret  the  spherules  as 
the  result  of  proteid  absorption.  It  is  well  known  that  a  certain  amount  of 
proteid  is  absorbed  in  the  large  intestine,  and  it  has  recently  been  found, 
by  Dr.  J.  L.  Bremer,  that  beneath  its  epithelium,  reticular  appearances 
similar  to  those  in  the  small  intestine  occur  after  proteid  digestion. 

The  muscularis  mucosae  of  the  small  intestine  consists  of  an  inner 
circular  and  an  outer  longitudinal  layer  of  smooth  muscle.  The  submucosa 
is  of  loose  fibrous  connective  tissue  with  few  elastic  fibers.  The  mus- 
cularis includes  an  inner  circular  layer  of  smooth  muscle  fibers,  and  a  much 
thinner  outer  longitudinal  layer.  Between  them  is  a  narrow  but  important 
band  of  connective  tissue.     Numerous  elastic  fibers  are  found  not  only 


MESENTERY.  211 

on  the  surfaces  of  the  muscle  layers  but  also  in  their  interior.  Their 
abundance  is  directly  proportional  to  the  thickness  of  the  musculature. 

The  serosa  consists  of  connective  tissue  which  is  covered  with  meso- 
thelium  except  along  the  line  of  attachment  between  the  intestine  and  its 
mesentery.  As  shown  in  the  diagram,  Fig.  237,  the  mesentery  is  a  thin 
layer  of  connective  tissue  bounded  on  either  side  by  mesotHelium,  which 
serves  to  suspend  the  intestine  from  the  median  dorsal  line  of  the  body 
cavity.  It  is  present  unless  adhesions  occurring  in  the  course  of  develop- 
menf'have  destroyed  it,  and  in  the  small  intestine  such  adhesions  involve 
only  a  part  of  the  duodenum.  At  the  root  of  the  mesentery  (the  portion 
attached  to  the  trunk  of  the  body)  the  mesothelium  extends  laterally  and 
with  the  underlying  connective  tissue  forms  the  parietal  peritonaeum. 
The  tunica  serosa  of  the  intestine  and  the  lateral  parts  of  the  mesentery 
constitute  the  visceral  peritonaeum  (this  term  being  applied  especially 
to  the  former).  The  mesothelium  of  the  entire  peritonaeum  consists  of 
fiat,  polygonal  cells  shown  in  surface  view  in  Fig.  238.  The  outer  por- 
tions of  the  cells  fit  closely,  but  the  deeper  parts,  containing  the  nuclei, 
are  joined  by  intercellular  bridges.  Beneath  the  epithelium  there  is 
fibrillar  connective  tissue  containing  abundant  elastic  networks  parallel 
with  the  surface,  and  having  plasma  cells  and  other  free  forms  in  its 
meshes.  These  cells  are  found  especially  along  the  blood  vessels. 
The  connective  tissue  layer  is  denser  in  the  parietal  than  in  the  visceral 
peritonaeum.  In  places  where  the  peritonaeum  is  freely  movable  there 
is  a  subserous  layer  of  loose  fatty  tissue,  but  there  is  no  distinct  subse- 
rous layer  in  the  intestine.  The  mesothehal  layers  on  the  opposite  sides 
of  the  mesentery  are  so  close  together  that  they  may  both  be  seen  in  a  sur- 
face preparation  by  changing  the  focus,  or  even  simultaneously  as  at  X  in 
Fig.  238.  The  connective  tissue  between  them  is  thin  except  where  it 
surrounds  the  larger  blood  and  lymphatic  vessels  and  nerves  which  pass 
through  the  mesentery  to  and  from  the  intestine. 

Blood  vessels  0}  the  small  intestine.  The  arteries  pass  from  the  mesen- 
tery into  the  serosa  in  which  their  main  branches  tend  to  encircle  the 
intestine.  Smaller  branches  from  these  pass  across  the  muscle  layers  to 
the  submucosa  in  which  they  subdivide  freely  (Fig.  239,  A).  In  crossing 
the  muscle  layers  they  send  out  branches  in  the  intermuscular  connective 
tissue.  These  and  the  arteries  of  the  serosa  and  submucosa  supply  the 
capillary  networks  found  among  the  muscle  fibers.  The  capillaries  are 
mostly  parallel  with  the  muscles.  From  the  submucosa  the  arteries  invade 
the  mucosa  forming  an  irregular  capillary  network  about  the  glands,  and 
sending  larger  terminal  branches  into  the  villi.  There  is  usually  a  single 
artery  for  a  villus  and  it  has  been  described  as  near  the  center  with  the  veins 


212 


HISTOLOGY. 


cm. 

i.e. 
l.m. 
s. 


A 


B  C 

Fig.  239. 
A,  Diagram  of  the  blood  vessels  of  the  small  intestine  ;  the  arteries  appear  as  coarse  black  lines,  the 
capillaries  as  fine  ones,  and  the  veins  are  shaded  (after  Mall).  B,  Diagram  of  the  lymphatic  vessels 
(after  Mall).  C,  Diagram  of  the  nerves,  based  upon  Golgi  preparations  (after  Cajal).  The  layers  of 
the  intestine  are  m.,  mucosa  ;  m.  m.,  muscularis  mucosae  ;  s.  ttl.,  submucosa  ;  c.  rtl.,  circular  muscle  ; 
i.  c,  intermuscular  connective  tissue  ;  I.  m.,  longitudinal  muscle  ;  s.,  serosa,  c.  I.,  central  lymphatic  ; 
n.,  nodule ;  S.  pi.,  submucous  plexus  ;  m.  pi.,  myenteric  ple.xus. 

at  the  periphery  (Fig.  239),  or  on  one  side  of  the  villus  with  a  vein  on  the 
other.    The  network  of  blood  vessels  in  the  vilh  is  very  abundant  as  shown 


Vein. 


Tunica  propria.  ~ 


— ^^rjrtsi: 


.Muscularis  mucosae. 


.Submucosa. 

Fig.  240.— Vertical  Section  of  thk  Mucous  Me.mbrane  of  the  Human  Jejunum.    X  50. 
The  blood  vessels  are  injected  with  Berlin  blue.    The  vein  of  the  first  villus  on  the  left  is  cut  transversely. 


in  Fig.  240.     The  veins  branch  freely  in  the  submucosa  and  pass  out  of  the 
intestine  beside  the  arteries.     The  muscularis  mucosae  has  been  described 


VESSELS    OF    THE    SMALL    INTESTINE. 


213 


asJQrniirig--a__sp]iincterjnus^^  No  valves 

occur  until  the  veins  enter  the  tunica  muscularis;  there  they  appear,  and 
continue  into  the  collecting  veins  in  the  mesentery.  They  are  absent  from 
the  large  branches  of  the  portal  vein  which  receives  the  blood  from  the 
intestines. 

Lymphatic  vessels.  The  intestinal  lymphatics  [lacteals]  appear  as 
central  vessels  within  the  villi  (Fig.  239,  B).  Each  villus  usually  contains 
one,  which  ends  in  a  blind  dilatation  near  its  tip ;  soriietimes  there  are  two 
or  three  which  form  terminal  loops.     In  some  stages  of  digestion  the  dis- 


Villus. 


Intestinal  glands. 


Submucosa. 


Muscularis 
mucosae. 


^ 


Lymph  nodules. 


Circular        Longitudinal 
laver  laver 


of  the  muscularis. 
Fig.  241. — Transverse  Section  of  Aggreg.a.te  Nodules  of  the  Small  Intestine  of  .a.  Cat. 
The  crests  of  four  nodules  were  not  within  the  plane  of  the  section.     X  lo. 


tension  of  these  lymphatics  is  very  great  and  their  endothehum  is  easily 
seen  in  sections.  When  collapsed  they  are  hard  to  distinguish  froni  the 
surrounding  reticulum.  Small  lateral  branches  and  a  spiral  prolongation 
of  the  central  lymphatic  have  been  found  by  injection,  but  these  may  be 
tissue  spaces.  The  lymphatics  branch  freely  in  the  submucosa  and  have 
numerous  valves.  They  cross  the  muscle  layers,  spreading  in  the  inter- 
muscular tissue  and  the  serosa,  and  pass  through  the  mesentery  to  the 
thoracic  duct. 

Lymphoid  tissue.     The  lymphoidtissue  of  the  intestine  occurs  pri- 


214 


HISTOLOGY. 


marily  in  the  tunica  p^ro^riaj^^andm^hree  forms^-— diffuse  lym^tLQid..iissue»- 
■^litary  nodules,  land  aggregate  nodules.  Solitary  nodules  are  seen  in 
Figs.  232  and  244.  The  latter  shows  how  the  nodule  which  arises  in  the 
propria  may  extend  through  the  muscularis  mucosae  and  spread  in  the 
submucosa,  thus  being  as  a  whole,  flask  shaped  or  pyriform.  A  per- 
ipheral section  of  such  a  nodule  may  present  only  the  part  beneath  the  mus- 
cularis mucosae.  The  nodules  are  surrounded  by  small  vessels,  the  lym- 
phatics being  drawn  in  Fig.  239,  B.  Blood  vessels  may  make  a  similar 
net,  and  penetrate  the  outer  portion  of  the  nodule.  The  germinative  cen- 
ters are  similar  to  those  in  the  lymph  glands. 


f  ■ 


«  B 

Fig.  242. 
A,  Surface  view  of  the  plexus  myentericus  of  an  infant.     X  50.     g.  Groups  of  nerve  cells  ;  f,  layer  of  circu- 
lar muscle  fibers  recognized  by  their  rod-shaped  nuclei.     B,  Surface  view  of  the  plexus  submucosus  of 
the  same  infant.    X  50.    g,  Groups  of  nerve  cells ;  b,  blood  vessel  visible  through  the  overlying  tissue. 


Aggregate  nodules  [Peyer's  jpatches]  are  oval  macroscopic  areas, 
usually  from  i  to  4  cms.  long  but  occasionally  much  larger,  composed  of 
from  10  to  60  nodules  placed  side  by  side  (Fig.  241).  The  nodules  may 
be  distinct  or  blended  by  intervening  lymphoid  tissue.  They  distort  the 
intestinal  glands  with  which  they  are  in  relation,  and  immediately  above 
the  nodules  the  villi  are  partly  or  wholly  obliterated!  Thus  they  appear 
as  dull  patches  in  the  lining  oTlHe  freshly  opened  intestine.  There  are 
from  15  to  30  of  them  in  the  human  intestine  (rarely  as  many  as  50  or  60) 
and  they  occur  chiefly  in  the  lower  part  of  the  ileum  on  the  side  oppasite_ 
tlielnesentery,.  A  few  occur  in  the  jejunum  and  the  distal  part  of  the  duo- 
denum. In  the  vermiform  process  diffuse  aggregate  nodules  are  always 
present,  but  they  do  not  occur  elsewhere  in  the  large  intestine. 


NERVES    OF   THE    SMALL   INTESTINE. 


21=; 


Nerves.  The  small  intestine  is  supplied  by  branches  of  the  superior 
mesenteric  plexus  of  the  sympathetic  system.  This  plexus  is  ventral  to 
the  aorta,  and  sends  branches  through  the  mesentery  into  the  serosa. 
The  manner  in  which  they  penetrate  the  other  layers,  forming  the  myen- 
teric plexus  [Auerbach's  plexus]  in  the  intermuscular  connective  tissue,  and 
the  submucous  plexus  [Meissner's  plexus]  in  the  submucosa  is  shown  in 
Fig.  239,  C.  In  surface  view,^  obtained  by  stripping  the  layers  apart, 
these  plexuses  are  seen  in  Fig.  242.  Their  branches  supply  the  smooth 
muscle  fibers.  From  the  submucous  plexus  the  nerves  extend  into  the  vilH, 
where  nerve  cells  have  been  detected  by  the  Golgi  method  (Fig.  239,  C) ; 
it  has  been  suspected,  however,  that  some  of  these  'nerve  cells'  are  portions 
of  the  reticular  tissue.  Their  terminations  require  further  investigation. 
Most  of  the  intestinal  nerves  are  nonmedullated  but  they  include  a  few 
large  medullated  fibers  said  to  have  free  endings  in  the  epithelium. 


Fig.  243.— Transverse  Section  of  the  Human  Vermiform  Process.    X  20.    (Sobotta.) 

Note  the  absence  of  villi  and  the  abundance  of  nodules.    Clear  spaces  in  the  submucosa  are  fat  cells. 

Onb'  a  part  of  the  circular  layer  of  the  muscularis  has  been  drawn. 


Large  Intestine — Vermiform  Process. 
The  entire  large  intestine  is  characterized  by  the  presence  of  intestinal 
glands  associated,  with  the  absence  of  villi^    In  human  embryos  of  from 


2l6 


HISTOLOGY, 


4  to  6  months  there  are  vilh  in  the  large  intestine,  but  they  disappear  be- 
fore birth,  by  becoming  flattened  out.  The  vermiform  process  is  distin- 
guished from  the  colon  by  its  small  diameter  and  by  the  abundance  of 
lymph  nodules  in  its  tunica  propria.  They  are  often  confluent  (Fig.  243). 
In  old  age  the  lumen  of  the  vermiform  process  is  frequently  obhterated; 
this  has  been  recorded  in  50%  of  persons  over  60  years  old,  and  appears 
to  be  a  normal  retrogression.  The  epithelium  with  its  glands,  and  the 
nodules  disappear  and  are  replaced  by  an  axial  mass  of  fibrous  tissue. 
This  is  surrounded  by  the  unaltered  submucosa,  muscularis,  and  serosa. 

Large  Ixtestixe — Caecum  axd  Colon. 
The  intestinal  glands  of  the  caecum  and  colon  are  longer  than  those 
in  the  small  intestine, — sometimes  twice  as  long  (0.4-0.6  mm.).     They 
contain  more  goblet  cells,  but  cells  of  Pancth  arc  absent.     Striated  cutic- 


Glands. 


MM 


/".•"•■'v'-"-','-.  %-■"*    Xr\—  Epithe- 
f   '      ';.*  '    ' ,     •       -^i^         Hum. 

•^  ■';■•■■••■■  }■':'■' ^ 


.  Tunica 
propria. 


■::',M^-->m 

■-:^'^::--mp 

_, Muscularis 

"  "    ~"   -_J  •■     ' — '^l^ 

mucosae. 

r  ~  —■ii..?^'^ 

-.•;          ■■;,, 

,  -  _,  -  'p  -r 

■'.-':        '~-' 

,  _ 

Submu- 

cosa. 

Fat  cells. 


Solitary  nodule  with  germinal  center. 


Fig.  244.— Vertical  Section  of  the  Mucous  Membrane  of  the  Descendixg  Colon 

OF  AN  Adult  Man.     X  80. 

Compare  the  length  of  the  glands  with  those  of  the  small  intestine  (Fig.  230),  from  the  same  individual 

and  drawn  under  the  same  masrnification. 


ular  borders  appear  near  the  outlets  of  the  glands  and  are  well  developed 
upon  the  columnar  cells  hning  the  intestinal  lumen.  Sohtary  nodules  are 
numerous,  especially  in  the  caecum.     Apart  from  the  muscularis,  the  re- 


LARGE    IXTESTIXE.  2I7 

maining  layers  resemble  those  of  the  small  intestine.  The  outer  longitudi- 
nal layer  of  the  muscularis  is  thin  except  where  its  fibers  are  gathered  into 
three  longitudinal  bands  or  taeniae,  nearly  equidistant  from  one  another, 
which  terminate  in  the  corresponding  layer  of  the  vermiform  process. 
The  latter  is  uniform  and  not  separated  into  taeniae.  Since  the  longi- 
tudinal bands  are  shorter  than  the  inner  layers  of  the  colon,  internal  trans- 
verse folds  are  produced,  called  plicae  semilunares.  Inasmuch  as  the 
circular  muscle  layer  is  included  in  them,  they  differ  from  the  plicae  cir- 
culares  of  the  small  intestine.  They  occur  at  considerable  inter\'als  and 
between  two  successive  phcae  the  wall  of  the  colon  exhibits  a  saccular 
dilatation  or  haiistrum.  The  valve  of  the  colon  (valvula  coli)  is  a  pair  of 
folds  or /fl&ia,  which  are  similar  in  structure  to  the  semilunar  folds ;  that  is, 
they  include  fibers  of  the  circular  muscle  layer,  but  the  shorter  layer  of 
longitudinal  fibers  passes  directly  from  the  ileum  to  the  colon  without 
entering  the  valves.  Figures  of  the  bands,  folds,  pouches,  and  valves  of  the 
colon  may  be  found  in  the  text-books  of  anatomy,  and  to  these  the  stu- 
dent should  refer.  The  serosa  of  the  colon  contains  lobules  of  fat  which^ 
form  pendulqus^^ojections  knov^n  as  appendices  epiploicae. 

Rectum  and  Axus. 

The  rectum  agrees  in  its  general  structure  with  the  colon,  and  has  even 
longer  glands  (0.7  mm.).  Its  fining  presents  transverse  folds  (plicae  trans- 
versales)  and  in  the  anal  region  there  are  several  longitudinal  folds,  the 
rectal  columns.  In  this  region  the  musculature  is  highly  developed.  The 
muscularis  mucosae  becomes  thicker  and  enters  the  colunms.  The  cir- 
cular layer  of  the  muscularis  terminates  as  a  special  accumulation  of  fibers, 
the  internal  sphincter  of  the  anus.  Just  beyond  it  is  the  external  sphincter, 
a  striated  muscle  of  the  perineal  group.  The  three  taeniae  of  the  colon 
unite  so  as  to  form  two  in  the  rectum,  a  ventral  [anterior]  and  a  dorsal 
[posterior],  but  by  the  development  of  fibers  between  them  the  longitudinal 
layer  becomes  essentially  complete  and  uniform.  It  terminates  by  joining 
the  internal  sphincter  and  neighboring  muscles,  and  by  ending  in  the  sub- 
epithehal  tissue. 

A  short  distance  within  the  internal  sphincter  the  simple  columnar 
epithelium  abruptly  becomes  a  thick  stratified  layer  \\ith  flat  outer  cells. 
Its  base  rests  upon  vascular  papillae.  The  rectal  glands  extend  for  a 
short  distance  into  the  region  of  stratified  epithehum.  The  circumanal 
glands  which  appear  as  modified  sweat  glands  occur  beyond  the  anus,  in 
the  skin. 

The  vessels  and  nerves  of  the  large  intestine  are  distributed  essentially 
as  in  the  small  intestine,  except  for  the  absence  of  aiIH.     The  great  abun- 


2l8 


HISTOLOGY. 


dance  of  veins  in  the  submucosa  of  the  anal  part  of  the  rectum  should  be 
noted  because  of  its  clinical  importance. 

Liver. 

The  liver  is  one  of  the  three  organic  glands  which  develop  from  the 
digestive  tube,  the  others  being  the  pancreas  and  the  lungs. 

Development  oj  the  liver.  The  liver  arises  as  a  clump  of  rounded 
masses  of  entodermal  cells  which  proliferate  from  the  ventral  surface  of  the 
"pharynx"  just  anterior  to  the  yolk  sac.  It  is  shown  in  the  diagram  Fig. 
245,  A.  The  liver  at  this  stage  lies  between  the  vitelline  veins,  in  the  con- 
nective tissue  which  extends  from  the  mesothelium  of  the  pericardial  cavity 
to  the  entodermal  layer  of  the  yolk  sac.     Since  this  connective  tissue  forms 


A  v.v.   int.  B  C 

Fig.  245.— Diagrams  of  the  Development  of  the  Liver. 

A,  The  condition  in  a  4.0  mm.  human  embryo.  B.  A  12  mm.  pig.  C,  The  arrangement  of  ducts  in  the 
human  adult,  c.  d..  Cystic  duct ;  c.  p.,  cavity  of  tlie  peritonaeum  ;  d.,  duodenum  ;  d.  c,  ductus  chole- 
dochus  ;  dia.,  diaphragm  ;  div.,  diverticulum  ;  f.  I.,  falciform  ligament ;  g.  b.,  gall  bladder ;  g.  0.,  greater 
omentum  ;  h.  d.,  hepatic  duct ;  ht.,  heart ;  int,  intestine ;  IL,  liver;  I.  0..  lesser  omentum  ;  m.,  medias- 
tinum ;  oe..  oesophagus  ;  p.  c.  pericardial  cavity;  p.  d.,  pancreatic  duct;  ph.,  pharynx;  p.  v.,  portal 
vein  ;  st.,  stomach  ;  tr.,  trabecula  ;  v.  C.  i.,  vena  cava  inferior  ;  v.  v.,  vitelline  vein  ;  y.  s.,  yolk  sac. 


a  septum  across  the  body,  separating  the  cavity  of  the  yolk  sac  from  that 
of  the  pericardium,  it  is  called  the  septum  transvcrsum.  With  further 
growth  the  liver  becomes  divisible  into  two  parts ;  first,  a  more  or  less  cylin- 
drical diverticulum  of  the  intestine  (Fig.  245,  B,  div.)',  and  second,  a  mass 
of  branched  columns  of  entodermal  cells,  the  hepatic  trabeculae,  which 
grow  out  from  the  diverticulum  and  form  the  essential  part  of  the  Hver 
(Fig.  245,  B,  tr.).  The  trabeculae  are  not  irregular  detached  islands  as  seen 
in  single  sections,  but  through  anastomosis  with  one  another  they  form  a 
single  complex  network  of  solid  cellular  cords.  At  first  they  are  connected 
with  the  diverticulum  by  several  strands  of  cells,  as  in  B,  but  later  all  of 
these  atrophy  and  disappear  except  one,  which  forms  a  permanent  com- 
munication between  the  trabeculae  and  the  diverticulum.     After  acquiring 


LIVER. 


219 


The  septum  is  bounded 


a  lumen  it  is  knc^TL  as  the  hepatic  duct,  C,  h.d.  The  diverticulum  becomes 
enlarged  at  its  distal  end  to  form  the  gall  bladder,  g.b.  This  has  a  tapering 
neck  leading  to  the  cystic  duct,  c.d.  After  receiving  the  hepatic  duct,  the 
diverticulum  forms  the  common  bile  duct  (ductus  choledochus)  which  enters 
the  duodenum.  (Just  before  the  entrance  it  is  joined  by  the  pancreatic 
duct,  p.d.)    ' 

From  its  development  the  Hver  is  seen  to  be  an  entodermal  organic 
gland  with  branched  and  anastomosing  terminal  pieces.  It  develops  in 
the  septum  in  close  relation  with  the  vitelline  veins.  Before  describing  the 
structure  of  the  adult  Hver  the  transformations  of  the  septum  and  of  the 
veins  should  be  considered. 

Transj  or  motion  of  the  septum  transversum. 
anteriorly,  that  is,  toward  the  head,  by  the  meso- 
thehum  of  the  pericardium  and  of  the  pleurae. 
Beneath  the  'mesotheHum,  striated  muscle 
spreads  out  in  the  septum  transversum,  produc- 
ing the  diaphragm.  Ventrally  the  septum  ex- 
tends from  the  Hver  to  the  subcutaneous  tissue 
of  the  abdominal  waU;  dorsally  it  passes  from  the 
Hver  to  the  lesser  curvature  of  the  stomach  and 
the  first  part  of  the  duodenum.  Posteriorly,  as 
sho"^!!  in  Fig.  245,  B,  a  rupture  occurs  through 
it  so  that  the  cavity  of  the  peritonaeum  extends 
from  side  to  side  between  the  diverticulum  and 
the  smaU  intestine.  The  gaU  bladder  is  thus 
provided  with  a  serous  coat,  similar  to  that  of  the 
intestine,  and  it  extends  over  the  sides  of  the 
Hver.  It  forms  the  lateral  walls  of  the  connec- 
tive tissue  layers  passing  from  the  Hver  to 
phragm  and  ventral  body  wall 


Thf.  Left  Side  of 
AN  Adult  Liver.  (Com- 
pare with  Fig.  244,  B.) 

d.  C,  Ductus  choledochus;  g.b., 
gall  bladder;  f.  1.,  falciform 
ligament;  I.  o.,  lesser  omen- 
tum; I.  t.  I.,  left  triangular 
ligament;  p.  v..  portal  vein; 
r.  I.,  round  ligament;  v.  C.  i., 
vena  cava  inferior. 


the  stomach,  the  dia- 
These  connective  tissue  layers  with  their 
mesotheHal  covering  are  mesenteric  structures  known  as  the  ligaments  of 
the  liver.  The  primary  ones  which  represent  the  original  septum  trans- 
versum are  the  falciform  ligament  between  the  Hver  and  ventral  abdominal 
waH  and  diaphragm,  in  the  median  plane ;  the  lesser  omentum  betv^een  the 
Hver  and  the  duodenum  and  lesser  curvature  of  the  stomach,  also  in  the 
median  plane  (the  blood  vessels  to  the  Hver  and  the  common  bile  duct  are 
within  the  lesser  omentum) ;  and  finaUy,  the  right  and  left  triangular  liga- 
ments between  the  liver  and  the  diaphragm.  The  triangular  Hgaments 
are  compressed  dorso-ventrally,  so  that"  their  line  of  attachment  to  the 
Hver  is  across  the  body  from  right  to  left.  The  relation  of  the  right  tri- 
angular Hgament  to  the  coronary  ligament  will  be  described  with  the 


220 


HISTOLOGY, 


blood  vessels.  Surrounding  the  en+ire  liver  close  to  the  hepatic  trabeculae, 
the  septum  transversum  produces  the  dense  fibrous  capsule  [capsule  of 
Glisson],  The  fibrous  capsule  is  covered  by  the  tunica  serosa  everywhere 
except  at  the  mesenteric  or  ligamentous  attachments.  Thus  the  trans- 
verse septum  produces  the  diaphragm,  the  falciform  and  triangular  liga- 
nients,  the  lesser  omentum,  the  fibrous  capsule  of  the  liver  and  the  con- 
nective tissue  portion  of  the  serosa.  It  also  gives  rise  to  the  connective 
tissue  found  within  the  liver. 

Development  of  the  veins  0}  the  liver.  x\s  seen  in  Fig.  245,  A,  the  liver 
at  once  comes  into  close  relation  with  the  vitelline  veins.  The  latter  branch 
about  the  ramifications  of  the  hepatic  trabeculae  producing  sinusoids 
(described  on  page  125).  At  first  there  are  two  vitelline  veins,  a  right  and 
a  left,  one  on  either  side  of  the  intestinal  tract.  They  anastomose  with 
one  another  dorsal  to  the  duodenum  as  shown  in  Fig.  247,  A.    Ventral  to  a 

more  distal  portion  of  the  duodenum  they  fuse 
and  thus  proceed  to  the  yolk  sac.  By  the 
obliteration  of  the  portions  of  these  veins  in- 
dicated in  Fig.  247,  B,  the  portal  vein  is  iormQd. 
and  its  adult  relations  to  the  duodenum  are  ex- 
plained. It  receives  the  blood  from  the  intes- 
tines, stomach,  spleen  and  pancreas,  through 
branches  which  develop  later,  and  conveys  it  to 
the  liver.  It  follows  the  hepatic  duct  and  its 
branches  into  the  liver,  where  it  is  resolved  into' 
sin-usoids.  These  unite  anteriorly  to  form  that 
part  of  the  vena  cava  inferior  which  passes 
from  the  liver  to  the  right  atrium  of  the  heart.  As  may  be  seen  in  Fig. 
245,  this  part  of  the  vena  cava  is  essentially  a  persistent  portion  of  the  vi- 
telline veins.  Three  other  veins  connect  with  the  vitelline  sinusoids  in  the 
li\-er,  namely  the  right  suhcardinal  vein  which  forms  a  large  part  of  the 
vena  caA'a  inferior,  and  the  right  and  left  umbilical  veins. 

The  distal  portion  of  the  vena  cava  inferior  is  derived  from  the  right 
vein  of  a  pair  which  pass  up  the  back  near  the  aorta;  their  position  may 
be  understood  since  their  anterior  parts  persist  as  the  azygos  and  hemiazygos 
veins  of  the  adult.  In  the  embryo  these  veins  are  prolonged  posteriorly 
(in  part)  as  the  right  and  left  subcardinal  veins,  shown  in  the  cross  section, 
Fig.  248.  This  figure  indicates  also  that  the  liver  fuses  with  the  dorsal 
wall  of  the  abdominal  cavity  on  the  right  side  of  the  body  (at  x).  No  cor- 
responding adhesion  occurs  on  the  left.  After  the  fusion  has  taken  place, 
the  right  subcardinal  vein  anastomoses  with  the  hepatic  sinusoids,  and  all 
of  the  blood  from  the  limbs  which  it  formerlv  took  to  the  heart  bv  wav  of 


Fig.  247. 
The  formation  of  the  portal  \eii) 
p.  v.,  from  the  right  and  left 
vitelline  veins,  r.  v.  and  I.  v.  : 
du.,  duodenum;  li.,  liver;  s. 
m.  v.,  superior  mesenteric  vein. 


LIVER. 


221 


the  azygos  vein,  now  passes  to  the  vitelHne  portion  of  the  inferior  vena  cava. 
The  original  connection  between  the  right  subcardinal  vein  and  the  azygos 
vein  is  destroyed.  In  short  the  vena  cava  inferior  represents  a  combina- 
tion of  different  veins.  The  part  from  the  hepatic  sinusoids  to  the  heart 
is  derived  from  the  original  vitelline  veins  as  shown  in  Fig.  245,  B.  The 
distal  part  includes  another  vein  secondarily  joined  to  the  former  through 
the  adhesion  of  the  right  lobe  of  the  liver  to  the  dorsal  abdominal  wall.  This 
adhesion  is  of  fundamental  importance.  It  appears  on  the  dorsal  surface 
of  the  adult  liver  (Fig.  249,  c.  /.)  as  a  somewhat  triangular  area,  uncovered 
by  peritonaeum,  known  as  the  coronary  ligament.  This  hgament  is  in 
reahty  an  extension  downward  of  the  right  triangular  hgament.  It  is 
usually  described,  however,  as  interposed  between  the  right  and  left  tri- 


f.l.    v.um. 

Fig.  24S.  —  Cross  Section  of  a  Mammalian 
Embryo,  to  show  the  Adhesion,  x,  be- 
tween THE  Right  Lobe  of  the  Liver  and 
THE  Dorsal  Abdominal  Wall. 

ao.,  Aorta;  f.  c,  fibrous  capsule  and  serosa;  f.  I,, 
falciform  lig:ament ;  g.  o.,  greater  omentum; 
I.  0.,  lesser  omentum  ;  I.  s-C.  v.,  left  subcardinal 
vein  ;  o.  b.,  omental  bursa  ;  r.  s-C.  v.,  right  sub- 
cardinal  vein  ;  st.,  stomach  ;■,  v.  urn.,  left  umbil- 
ical vein. 


.l.t.l 


yV.c.'i.     o.b. 


r.t.l. 


Fig. 


!49.— Dorsal  Surface  of  the  Adult 
Liver. 


C.  I..  Coronary  ligament;  f.  I.,  falciform  ligament; 
g.  b.,  gall  bladder ;  I.  o.,  lesser  omentnm  ;  1. 1. 1., 
left  triangular  ligament;  o.b.,  caudate  lobe 
bounding  the  omental  bursa  ventrally;  p.  v., 
portal  vein  ;  r.  I.,  round  ligament ;  r.  t.  I.,  right 
triangular  ligament ;  v.  c.  i.,  vena  cava  inferior. 


angular  ligaments.  The  coronary  adhesion  leads  to  the  formation  of  the 
foramen  epiploicum  [of  Winslow]  and  bounds  the  superior  recess  of  the 
omental  bursa  (Figs.  248  and  249,  o.b.).  The  foramen  and  bursa  are 
further  described  in  text  books  of  anatomy.  The  development  of  the  vena 
cava  is  figured  with  the  veins  of  the  Wolffian  body,  on  page  246. 

The  umbilical  veins,  one  on  either  side,  pass  from  the  umbiKcal  cord 
through  the  lateral  abdominal  walls  to  the  liver,  which  they  enter  through 
the  septum  transversum.  They  connect  with  the  sinusoids.  The  right  um- 
bilical vein  subsequently  becomes  small  and  loses  its  connection  with  the 
liver  (Fig.  248).  The  left  umbilical  vein  is  gradually  shifted  to  the  median 
ventral  Hne  and  passes  from  the  umbilicus  to  the  liver  along  the  free  margin 
of  the  falciform  ligament.  It  maintains  a  distinct  channel  across  the  Hver, 
apparent  on  the  lower  hepatic  surface.     From  the  ventral  margin  to  the 


222 


HISTOLOGY. 


lesser  omentum,  the  umbilical  vein  is  on  the  left  of  the  gall  bladder  from 
which  it  is  separated  by  the  quadrate  lobe.  After  the  umbiHcal  cord  is 
severed  the  vein  becomes  a  fibrous  strand,  known  as  the  round  ligament  of 
the  liver,  Fig.  249,  r.  L  It  extends  from  the  umbilicus  along  the  free  margin 
of  the  falciform  hgament,  and  under  the  liver  to  the  lesser  omentum.  From 
this  point  to  the  vena  cava  the  umbilical  vein  is  called  the  ductus  venosus. 
It  follows  the  hepatic  attachment  of  the  lesser  omentum,  and  there  it  forms 
after  birth,  the  ligament  of  the  ductus  venosus. 

Development  of  the  connective  tissue  and  hepatic  artery.  The  history 
of  the  Hver  has  been  described  to  that  point  where  it  consists  of  a  great  net- 
work of :  entodermal  trabeculae  connected  with  the  intestine  by  a  single 


,-v;\i;Vj:f,fry7.'.'- 


Branch  of  portal  vein. 


Central  veins. 


/  LariJ^e  interlobular  bile  duct. 


Interlobular  connective 
tissue. 


Central  vein. 


■  /^''.V 

■M,: 


Fig.  250.— From  a  Tangential  Section  of  the  Human  Liver.     X  40. 
The  three  central  veins  in  cross  section  mark  the  centers  of  three  lobules,  which  are  not  sharply  separated 
i  —  at  the  periphery  from  their  neighbors.     Below  and  at  the  right  the  lobules  are  cut  obliquely  and 
their  boundaries  are  not. 


duct.  Its  trabeculae  are  separated  by  a  very  small  amount  of  connective 
tissue  from  the  endothelium  of  the  sinusoids.  The  latter  are  essentially 
subdivisions  of  the  portal  vein  which  reunite  in  the  vena  cava  inferior. 
Later  in  development  the  connective  tissue  around  the  principal  branches 
of  the  portal  vein  increases  so  as  to  be  conspicuous;  to  a  less  extent  that 
which  surrounds  the  main  hepatic  branches  of  the  vena  cava  is  also  in- 
creased. Since  the  portal  branches  are  associated  with  the  bile  ducts  they 
may  be  distinguished  from  the  caval  branches.  Moreover  the  hepatic 
artery  which  develops  rather  late,  grows  into  the  connective  tissue  along 
the  bile  ducts.  It  suppHes  the  fibrous  capsule  and  the  connective  tissue 
layers  with  capillaries,  which  empty  into  the  adjacent  sinusoids  and  into 
the  portal  capillaries  limited  to  the  connective  tissue.     Thus  there  is  { a 


LIVER. 


223 


capillary  circulation  in  the  liver,  in  addition  to  the  sinusoidal,  but  the 
former  is  essentially  confined  to  the  connective  tissue. 

Microscopic  appearance  of  the  adult  liver.  In  sections  of  the  adult 
human  liver  there  will  be  seen  clumps  of  connective  tissue  which  contain 
branches  of  the  portal  vein,  hepatic  artery,  and  bile  ducts,  the  last  being 
easily  distinguished  by  their  columnar  or  cuboidal  epithelium  (Fig.  250). 
Lymphatic  vessels  and  nerves  (non-medullated  fibers  but  no  ner^^e  cells) 
may  also  be  found  in  this  connective  tissue.  There  is  a  tendency  for  the 
connective  tissue  areas  to  anastomose  with  one  another.     Pathologically 


bee 


-itT^^v-j-TrTX3^ 


^^ 


ff- 


,#^v^,:. 


f 

Fig.  251.— Liver  of  a  Pig.     (Radasch.) 

The  lobules  have  artificially  shrunken  from  the  interlobular  tissue,  a  ;    b,  bile  duct  ;  c,  hepatic  arterj- ;  d, 
interlobular  vein  (a  branch  of  the  portal)  ;    e,  trabeculae;  f,  central  vein. 

in  man,  but  normally  in  certain  animals,  as  in  the  pig,  this  anastomosis  is 
complete  and  polygonal  areas  of  hepatic  trabeculae  are  thus  made  promi- 
nent (Fig.  251).  These  are  the  lobules  of  the  fiver,  and  the  connective 
tissue  around  them  is  the  interlobular  connective  tissue,  containing  inter- 
lobular veins  (the  branches  of  the  portal).  In  the  center  of  each  lobule  is 
a  large  sinusoid,  the  central  vein  (sometimes  there  are  two).  Toward  it 
the  sinusoids  converge  from  the  interlobular  veins  on  all  sides  (Fig.  252), 
and  from  it  the  hepatic  trabeculae  radiate.  The  central  veins  open, 
usually  at  right  angles,  into  the  larger  sublobular  veins  (Fig.  253)  The 
latter,  being  derived  from  sinusoids,  have  notably  Httle  connective  tissue 


224 


HISTOLOGY. 


Two  bile  ducts  — 
in  cross 
section. 


Sinusoids 


Bile  duct    _ 
cut 
lengthwise. 


in  their  walls.     The  sublobular  veins  unite  to  form  the  hepatic  tributaries 

of  the  vena  cava 
inferior.  The  path 
of  the  blood  through 
the  liver  is  then 
briefly  as  follows: 
portal  vein,  inter- 
lobular veins,  sinus- 
oids, central  veins, 
sublobular  veins, 
hepatic  veins,  vena 
cava  inferior.  The 
hepatic  artery- 
through  capillaries 
connects  with  the 
interlobular  veins 
and  with  the  sinus- 
oids at  the  periph- 
ery of  the  lobules. 
Certain  patholog- 
ical conditions  sug- 
gest that  the  cells 
near  the  center  of 

the   lobules    are    not   as   well   nourished    as    those    at    the   periphery. 
Hepatic  cells.     The  hepatic  cells  are  arranged  in  anastomosing  tra- 


Ftg.  252.— From  a  Section  of  the  Human  Adult  Liver 
Lnjected  through  the  Portal  Vein. 


Hepatic  lobules. 


Interlobular  connective 
tissue. 


Central  (intralobular) 
veins. 


Sublobular  vein. 


Fig.  253.— From  a  Vertical  Section  of  a  Cat's  Liver,  Lnjected  through 
THE  Vena  Cava  Inferior. 

The  central  veins  and  the  sublobular  vein  into  which  they  empty  are  cut  longitudinally.     X  15. 


LIVER. 


225 


beculae  as  shown  in  Fig.  254.  Near  the  central  veins  they  form  terminal 
loops.  The  cells  composing  the  trabeculae  are  polygonal  or  cuboidal  with 
an  exoplasmic  layer  which  sometimes  resembles  a  cell  membrane.  The 
cells  contain  round  nuclei  which  are  variable  in  their  staining  capacity; 


True  meshes. 


Lateral  branches  of  bile  capillaries. 


Bile  capil- 
lary in  the 

anastomosis 
between 

two  hepatic 
trabeculae. 


Nuclei  of 

hepatic 

cells. 


Portion  of  a  central  veui. 


Fig.  254. — From  a  Cross  Section  of  a  Human  Hep.a.tic  Lobule.    X  300. 

Golgi  preparation.     The  boundaries  of  the  hepatic  cells  could  not  be  seen.     The  black  dots  are 

precipitates  of  the  silver. 

they  may  be  dark  or  pale.  Often  a  cell  contains  two  nuclei,  and  rarely 
large  cells  with  several  nuclei  produced  by  amitosis,  have  been  found. 
The  protoplasm  is  granular.  It  often  contains  brown  pigment,  especially 
toward  the  central  vein.  Near  the  periphery  of  the  lobule  the  cells  may 
contain  fat  vacuoles  of  varying  size,  found  normally  in  well  nourished 
15 


226 


HISTOLOGY. 


mm 


individuals.  Pathologically  the  vacuoles  may  be  large  and  have  a  wider 
distribution.  Glycogen  (page  51)  occurs  in  granules  and  larger  masses, 
especially  after  abundant  meals.  In  the  fasting  condition  the  cells  are 
relatively  small,  dark,  and  obscurely  outlined,  but  during  digestion  they 

become  larger,  Avith  a  clear  central  part 
and  dark  periphery  (Fig.  255).  In  man 
both  conditions  may  be  found  in  one  liver. 
The  bile,  secreted  by  the  hepatic  cells, 
frequently  contains  drops  of  fat  and  pig- 
ment granules  such  as  occur  within  the 
cells.  It  is  eliminated  through  the  bile 
capillaries. 

Bile  capillaries.  The  hepatic  trabec- 
ule, as  they  proliferate  from  the  diver- 
ticulum, form  a  network  of  solid  cords. 
Within  the  cords  a  slender  lumen  de- 
velops later,  apparently  beginning  at  the  periphery  of  the  lobule 
and  extending  toward  the  center.  It  causes  such  cords  as  consist  of  only 
two  rows  of  cells  to  resemble  the  tubules  of  other  glands,  as  shown  in  the 
diagram  Fig.  256.  In  uninjected  sections  the  lumen,  if  cut  across,  appears 
as  a  minute  circle  midway  along  the  line  of  contact  between  two  hepatic 


Fig.  255. — Liver  Cells  of  Man.     x  560. 

A,  Isolated  liver  cells  containing  smalk-r 
and  larger  fat  drops,  f.;  b.,  imprint 
from  contact  with  a  blood  vessel.  B, 
From  a  section  ;  1,  empty  ceils ;  2,  cells 
filled  with  secretion. 


\._i^^        lumen   (bile 
&     .^  capillar}-). 


/    r^ 


Blood  vessels. 


Fig.  256.— Diagram  of  a  Tubulk  of 
THE  Liver  (Disregarding  the 
Anastomoses  with  .Adjoining 
Tubules). 


Sinusoids. 


Fig.  257.— Section  of  the  Liver  of  a  Rabbit  with  the 
Bile  Capillaries  Lnjected.     X  560.     (^N'o/ a  Diagram.) 
Two  of  the  cells  are  each  in  contact  with   four  sinusoids 

(1,  2,  3,  4  I  and  four  bile  capillaries.    X,  a  bile  capillary 

where  three  cells  meet. 


cells.  Its  sharp  contour  is  ascribed  to  a  cuticular  formation  belonging  to 
the  cells  which  bound  it.  In  longitudinal  view  it  appears  as  a  dark  inter- 
cellular line  suggesting  a  cell  wall.  Both  views  are  sho\Mi  in  the  injected 
specimen  Fig.  257,  where,  however,  those  seen  longitudinally  seem  to  dis- 


LIVER. 


227 


Bile  capillaries. 


Sinusoids. 
Fig.  25S.— From  a  Rabbit's  Liver.    X  57°- 

The  cells  1,  2,  a  and  b,  are  cut  in  halves  :  their  four 
bile  capillaries  (including  I  and  II)  may  be  inter- 
cellular branches  of  trabecular  bile  capillaries 
shown  in  the  group  3,  4,  cand  d.  The  latter  are 
.seen  in  suiface  view,  the  plane  of  section  there 
being  between  the  cells.  The  actual  arrangement 
can  be  determined  only  by  reconstruction. 


regard  cell  boundaries;  this  is  because  they  he  in  intercellular  spaces  turned 
toward  the  observer,  the  cells  beneath  presenting  an  uncut  surface.  Some- 
times (as  at  x)  a  lumen  occurs  at  the  angle  where  three  hepatic  cells  meet, 
but  usually  sinusoids  are  found  at  the  comers  of  the  cells  and  as  seen  in  the 
figure,  a  lumen  tends  to  be  placed 
as  far  from  the  blood  vessels  as 
possible. 

A  bile  capillary,  as  the  lumen 
is  called,  follows  the  trabeculae, 
branching  and  anastomosing  as 
they  do  (Fig.  254).  Between  the 
hepatic  cells,  the  bile  capiharies 
send  off  branches  at  right  angles. 
These  intercellular  capillaries  are 
similar  in  diameter  and  structure 
to  \hQ  trabecular  capillaries.  They 
are  unbranched  and  end  bhndly 
before  reaching  the  vascular  sur- 
face of  the  cells  (Fig.  254).  In 
cases  of  pathological  obstruction 

of  the  bile  ducts,  however,  the  intercellular  capillaries  are  said  to  be 
prolonged  to  that  surface  and  to  rupture,  so  that  bile  enters  the  tissue 
spaces  and  the  vessels,  producing  jaundice. 

Intracellular  bile  capillaries  also  occur;  several  have  been  found  to 
extend  from  the  trabecular  capillary  into  a  single  hepatic  ceU.     As  seen  in 

Golgi  specimens  they  may  termi- 
nate in  knobs  interpreted  as  vacu- 
oles of  secretion  (Fig.  259).  Since 
neighboring  capillaries  are  free  from 
these  branches,  the  intracellular 
capillaries  are  regarded  as  phases  of 
functional  activity,  accompanying 
the  discharge  of  secretion.  They 
have  been  reported  as  forming  bas- 
kets similar  to  the  secretory  capil- 
laries of  parietal  cells.  In  studying 
intracellular  capillaries,  care  must  be  taken  to  exclude  surface  views  of 
intercellular  forms. 

Sinusoids  and  perivascular  tissue.  The  endotheHum  of  the  sinusoids 
is  separated  from  the  hepatic  cells  by  a  thin  layer  of  reticular  tissue.  With 
special  methods  this  tissue  is  seen  to  consist  of  innumerable  fine  fibers  free 


Bile  capillaries 
"  without  knobs. 


Bile  capillaries 
with  knobs. 


Fig.    259. — From   a    Golgi    Preparation    of 
THE  Liver  of  a  Dog.    X  490. 


228 


HISTOLOGY. 


from  elastic  elements.  The  meshes  of  the  reticular  tissue  are  drained  by 
the  lymphatic  vessels  of  the  capsule  and  interlobular  tissue;  the  reticular 
tissue  itself  contains  no  vessels.  Unlike  other  reticular  tissue,  that  of  the 
lobules  is  free  from  cells  in  its  meshes.  In  the  embryo,  however,  it  con- 
tains large  numbers  of  erythroblasts  and  is  for  a  time  an  important  source 
of  blood  corpuscles.  A  few  nerve  fibers  which  terminate  upon  the  hepatic 
cells,  have  been  found  in  it. 

The  endothelium  of  the  sinusoids  is  easily  penetrated  by  injections, 
which  spread  in  the  reticular  tissue,  and  even  enter  the  hepatic  cells.     The 


Branch  of  portal 
vein. 


Small  interlobu- 
lar bile-duct, 
cotitinuing  in 
bile  capilla- 
ries. 


Large  interlobu- 
lar bile-duct. 


Branch  of  hepat- 
ic arterv-. 


Bile  capillaries. 


kA>{>< 


Vi 


J 


Wall  of  the  central  vein. 
Fig.  260.— Golgi  Prepar.atio.n  of  the  Liver  of  a  Dog.     X  240. 


blood  vessels  are  not  supposed  to  extend  into  the  cells;  the  injection  mass 
probably  invades  the  trophospongium  or  other  intracellular  canals.  In 
chloride  of  gold  preparations  the  endothehal  cells  of  the  sinusoids  appear 
stellate  and  have  been  mistaken  for  connective  (reticular)  tissue  cells. 
They  are  phagocytic.  Often  they  are  called  the  stellate  cells  [of  Kupffer]. 
The  ducts  of  the  liver.  The  ducts  in  an  island  of  interlobular  con- 
nective tissue  drain  the  bile  capillaries  from  all  the  surrounding  lobules. 
If  lines  are  drawn  connecting  the  central  veins  with  one  another  they  will 
bound  areas  (structural  units)  comparable  with  the  lobules  of  other  organs: 


GALL    BLADDER.  229 

in  their  centers  the  ducts  are  found.  The  actual  connection  between  the 
trabeculae  and  bile  ducts  is  very  difficult  to  observe  in  ordinary  sections, 
although  it  is  easily  seen  after  the  ducts  have  been  injected,  or  in  Golgi 
preparations  (Fig.  260).  The  transition  from  hepatic  cells  to  the  low  cu- 
boidal  cells  of  the  small  ducts  occurs  abruptly  at  the  borders  of  the  lobule. 
The  cuticula  of  the  bile  capillaries  is  continuous  ^^dth  that  of  the  ducts. 
The  larger  interlobular  ducts  have  a  simple  columnar  epithelium.  They 
are  said  to  anastomose  with  one  another. 

The  cystic,  hepatic,  and  common  bile  ducts  all  have  a  simple  columnar 
epithelium,  containing  occasional  goblet  cells.  It  rests  on  an  elastic  tunica 
propria,  surrounded  in  turn  by  a  submucosa.  In  the  cystic  duct  the  mu- 
cosa is  throT\-n  into  coarse  transverse  folds,  containing  muscle  fibers,  kno^Mi 
as  the  spiral  valve.  In  the  hepatic  and  common  bile  ducts  especially, 
branched  mucous  glands  extend  into  the  connective  tissue  layer  (glandulae 
mucosae  hiliosae).  Outside  of  them  is  a  tunica  muscularis  consisting 
chieflv  of  circular  fibers.  These  form  a  sphincter  around  the  bile  duct  at 
the  duodenal  papiUa  (and  there  are  similar  sphincters  around  the  outlets 
of  the  pancreatic  ducts).  The  parts  of  the  ducts  exposed  on  the  under 
surface  of  the  liver  are  covered  by  a  serosa. 

In  the  gall  bladder  the  mucosa  forms  a  network  of  folds.  The  colum- 
nar epithehal  cells  are  twice  the  height  of  those  in  the  common  bile  duct. 
Goblet  cells  are  absent  and  glands  are  infrequent.  SoHtary  nodules  may 
be  found  in  the  mucosa.  The  muscular  layer  is  of  obhquely  circular  fibers 
in  a  plexiform  layer.  Among  them  are  groups  of  s^Tnpathetic  ner^'e  cells, 
which  innervate  the  muscle.  There  are  also  meduUated  nen-e  fibers  in  the 
gall  bladder  which  terminate  in  its  epithehum.  The  subserous  portion 
of  the  serosa  is  highly  developed  and  contains  large  hmphatic  vessels. 

The  vasa  aberrantia  of  the  fiver  are  bhnd  ducts  which  extend  beyond 
the  territorA-  of  the  trabeculae.  They  are  found  about  the  left  lobe,  and 
especially  around  the  vena  cava,  the  porta  hepatis  and  the  left  triangular 
figament,  and  represent  portions  of  the  fiver  from  which  the  hepatic  cells 
have  degenerated  and  disappeared. 

The  porta  hepatis,  meaning  'gate  of  the  fiver'  is  the  place  where  the 
vessels  enter  and  the  ducts  leave,  thus  corresponding  wfith  the  hilus  of 
other  organs.  There  the  lymphatic  vessels  and  the  nerves  are  very  numer- 
ous. The  latter,  chiefly  non-medullated,  form  networks  around  the 
vessels  and  ducts.  They  extend  into  the  capsule  and  interlobular  tissue, 
chiefly  supplying  the  blood  vessels.  Some,  however,  continue  into  the 
lobules  to  the  hepatic  ceUs.  The  l}Tnphatic  vessels  anastomose  freely  in 
the  capsule  and  in  the  interlobular  tissue,  these  sets  connecting  ydxYi  one 
another.     Thev  do  not  enter  the  lobules. 


230 


HISTOLOGY. 


Pancreas. 
The  pancreas  is  a  large  entodermal  gland  consisting  of  lobes  and 
lobules  and  resembling  in  its  general  stnicture^the  parotid  gland.     It  arises 

as  two  distinct  out- 
growths of  the  di- 
gestive tract,  as  seen 
in  Fig.  261,  A. 
The  smaller  of 
these,  called  the 
ventral  pancreas,  de- 
velops from  the  duc- 
t  u  s  choledochus 
near  its  intestinal 
orifice.  Its  duct, 
called  the  pancre- 
atic duct  [of  Wir- 
sung],  opens  beside 
the  common  bile 
The  papilla  is  a  hollow  eleva- 


FiG.  261.  —  A,  Diagram  of  the  Pancreas  from  a  15  mm.  Human  Em- 
bryo. B,  Dissection  of  the  Duodenum  and  Pancreas  of  an 
Adult.     (After  Schiomer.) 

a.  p.  d.,  Accessory  pancreatic  duct ;  c.  d.,  cystic  duct ;  d.,  duodenum  ;  d.  C. 
ductus  choledochus;  d.  p.,  dorsal  pancreas;  h.  d.,  hepatic  duct;  p., 
duodenal  papilla  ;  p.  d.,  pancreatic  duct ;  St.,  stomach  ;  v.  p.,  ventral 
pancreas. 


duct  at  the  base  of  the  duodenal  papilla 
tion  of  the  mucosa,  which 
has  been  spread  open  in 
Fig.  261,  B.  The  larger 
part  of  the  pancreas  grows 
out  separately,  from  the 
dorsal  wall  of  the  duode- 
num between  the  papilla 
and  the  stomach.  The 
duct  of  this  dorsal  pancreas 
is  the  accessory  pancreatic 
duct  [of  Santorini].  The 
dorsal  pancreas  fuses  with 
the  ventral  so  as  to  make  a 
single  gland  of  uniform 
structure,  the  former  pro- 
ducing its  body  and  tail, 
and  the  latter  contributing 
to  the  head.  The  two 
ducts  anastomose  as  shown 
in  Fig.  261,  B,  and  the  out- 


Blood  \essel 


.' :    \l\eoli. 


^^yt.< 


Cells 
of  the  island 


Fig.  262.— An  Island  of  the  Pancreas  with  the 
Surrounding  Alveoli,  from  an  .Adult.    X  400. 


let  of  the  ventral  duct  becomes  predominant.     The  intestinal  end  of  the 


PAXCREAS. 


231 


^^ 


>TtJ^iJ.i%M 


b^a^.  -Sv:  rj,  !Mi.«  >,?«■ 


accessory  duct  is  sometimes  obliterated,  but  it  may  remain  pervious  and 
be  of  clinical  importance  in  case  of  obstruction  of  the  main  duct.  It  opens 
about  an  inch  above  the  papilla.  (In  the  pig,  which  is  often  studied 
embryologically,  the  dorsal  pancreas  enters  the  duodenum  distal  to  the 
papilla;  its  duct  persists  whereas  that  of  the  ventral  pancreas  is  obhter- 
ated.) 

As  is  true  of  most  glands,  the  developing  tubules  of  the  pancreas  are 
at  iirst  soHd,  but  in  the  pancreas  alone  certain  portions  of  the  prohferating 
tubules  become  detached  from  the  rest,  forming  islands  of  soHd  cords  of 
ceUs.  These  islands  [of 
Langerhans]  were  not  found 
in  a  human  embryo  of  28  mm. 
(53  days)  but  have  been 
recorded  at  54  mm.  (73  days). 
They  are  then  round  or  oval 
masses  of  cells  rich  in  finely 
granular  eosinophiHc  proto- 
plasm, which  are  still  con- 
nected with  the  developing 
alveoh.  Later  they  become 
detached,  and  by  the  invasion 
of  capillaries  of  large  diam- 
eter they  are  irregularly  sub- 
divided into  cords  as  seen  in 
Fig.  262.  The  islands  are 
said  to  appear  first  in  the  tail 
and  body  of  the  pancreas,  and 
later  in  the  head  where  they 
are  always  relatively  fewer. 
In   an  early  stage  they  are 

at  the  periphery  of  the  lobules  which  are  bounded  by  abundant  connective 
tissue,  but  subsequently  they  are  surrounded  by  the  prohferating  alveoli 
which  reduce  the  connective  tissue  to  interlobular  septa  (Fig.  263).  It  is 
not  now  supposed  that  the  islands  arise  from  connective  tissue,  or  that  they 
are  produced  in  adult  Hfe  by  the  degeneration  of  alveoli.  The  islands  have 
neither  ducts  nor  lumen.  Their  secretion,  which  is  internal,  is  received 
by  the  blood  vessels.  It  is  essential  for  the  metabolism  of  sugar  as  shown 
by  experiment.  After  removal  of  the  pancreas,  sugar  appears  in  the  urine; 
on  the  other  hand  if  the  pancreatic  ducts  are  tied  the  alveoli  degenerate 
but  the  islands  remain  intact,  and  sugar  does  not  appear  in  the  urine. 
Thus  the  islands  constitute  an  organ  within  but  functionally  independent 
of  the  pancreas. 


Fig.  263. — Section  of  Human  Pancreas,  showing  Sev- 
eral Islands,  f. 

a,  Interlobular  connective  tissue  containing  an  interlobular 
duct,  c:   b,  capillarj';   d,  interlobular  duct;   e,  alveoli. 

(Radasch.) 


2^2 


HISTOLOGY. 


In  sections  of  the  adult  pancreas  the  islands  are  areas  from  ,07  to  0.3 
mm.  in  diameter,  occupied  by  cords  or  groups  of  polygonal  cells,  the  boun- 
daries of  which  are  often  indistinct. 
The  nuclei,  round  or  oval,  contain  chro- 
matin in  many  small  granules,  together 
Avith  a  few  larger  ones.  The  protoplasm 
is  finely  granular  and  in  certain  of  the 
cells  only,  it  is  said  to  stain  intensely  with 
saffranin.  Sometimes  the  protoplasm 
appears  reticular.  The  islands  may  be 
separated  from  the  alveoh  by  a  consider- 
able layer  of  connective  tissue  in  which 
the  elastic  elements  are  infrequent,  or  by 
a  thin  basement  membrane.  Sometimes 
even  the  latter  is  absent.  The  endo- 
thelium of  the  capillaries  is  surrounded 
by  a  very  small  amount  of  reticular  tissue. 
The  pancreatic  and  accessory  pan- 
creatic ducts  are  hned  with  simple  col- 
umnar epithelium  which  is  surrounded 
by  an  inner  dense,  and  an  outer  loose 
layer  of  connective  tissue.  The  latter 
contains  some  smooth  muscle  fibers 
which  are  gathered  into  sphincters  at  the  outlets  of  the  ducts.  Oc- 
casional   goblet    cells,    and    small    glands    resembling    mucous    glands, 


Alveoli. 


Tubule. 
Fig.  264. — Diagram  of  the  Panxreas. 


Cells  of 
the  al- 
veolus. 


Inter-  Inter- 

calated calated 

duct.  duct. 


Centroacinal  cells 


A  B 

Fig.  265. — From  Sections  of  a  Hl'man  Pancreas. 


Zvniogen  arranules. 


X  500. 


In  section  A  the  granules  are  wanting,  the  elements  of  the  intercalated  duct  are  flat  and  dark  ;  in  section 
B  the  granules  are  distinct,  the  cells  of  the  intercalated  duct  are  cubical  and  clear. 


PAXCREAS.  22,3 

have  been  found  in  the  mucosa.  The  epitheUal  cells  become  lower 
in  the  smaller  ducts,  and  are  cuboidal  or  flattened  parallel  with  the 
long  axis  in  the  intercalated  ducts.  There  are  no  secretor}*  ducts  in  the 
pancreas.  The  long  intercalated  ducts  terminate  in  the  alveoli  (or  acini; 
in  a  pecuhar  manner.  As  seen  in  Fig.  265,  the  cells  of  the  duct  seem 
prolonged  into  the  center  of  the  alveolus,  where  they  are  kno^^Ti  as  centro- 
acinal  cells.  In  development  the  duct  is  not  invaginated  into  the  alveo- 
lus, but  the  latter  develops  so  as  to  consist  of  two  layers,  only  the  outer  of 
which  produces  the  pancreatic  secretion.  Sometimes  the  inner  cells 
are  lacking.  The  lumen  of  the  intercalated  ducts  and  alveoh  is  very 
small  and  in  many  parts  of  a  section  it  cannot  be  seen.  Intercellular  se- 
cretory capillaries  extend  from  it  between  the  centro-acinal  cells  to  the  se- 
creting cells,  as  seen  in  Fig.  266.  They  may  be  prolonged  between  the 
latter,  but  thev  do  not  reach  the  basement  membrane. 


Inter- 
calated 
duct. 


Centroacinal  cells. 


Cells  of  the 
alveolus. 


Intercellular 
secretorj- 
capillary. 


Fig.  266.— a,  From  a  Section  of  the  Pancre.as  of  Adult  Man.  X  320;  B,  An  Interpretation 
OF  THE  Right  Lower  Portion  of  A. 

The  secreting  or  pancreatic  cells  are  mostly  arranged  in  alveoU  but  in 
part  they  form  tubules.  Toward  the  lumen  their  protoplasm  contains  a 
zone  of  coarse  granules  of  zymogen,  which  accumulate  while  the  ceU  is  in- 
active and  are  ehminated  during  secretion.  Apparently  they  are  trans- 
formed into  fluid  as  they  are  discharged,  for  they  are  not  found  free  in  the 
intestine.  In  fresh  specimens  the  granules  are  refractive  and  easily  seen, 
but  in  preserved  tissue  they  are  readily  destroyed  so  that  the  granular  zone 
appears  reticular.  The  granules  are  soluble  in  water,  and  are  darkened 
by  osmic  acid.  The  basal  protoplasm  of  the  pancreatic  cells  is  vertically 
striated.  It  contains  the  round  nucleus  with  coarse  masses  of  chromatin. 
Within  the  pancreatic  cells  there  have  been  found  'paranuclei'  of  unknown 
nature,  thought  to  be  functionally  important.  After  the  discharge  of 
secretion  the  cells  become  smaller  and  their  boundaries  more  distinct. 
The  pancreatic  cells  rest  upon  basement  membranes  containing  '  basket 
cells.' 


234 


HISTOLOGY. 


The  blood  and  lymphatic  vessels  and  the  nerves  resemble  those  of  the 
salivary  glands.  The  capillaries  have  notably  wide  meshes  so  that  con- 
siderable portions  of  the  alveoli  are  not  in  contact  with  them.  The  nerves 
end  around  the  blood  vessels,  ducts  and  pancreatic  cells.  They  are  chiefly 
nonmedullated  sympathetic  fibers  from  the  coeliac  plexus,  associated  with 
scattered  nerve  cells  mthin  the  pancreas.  Lamellar  corpuscles  may  be 
found  in  the  connective  tissue. 


Development  of  the  Respiratory  Tract. 
The  respiratory  system,  consisting  of  the  larynx,  trachea,  bronchi, 
and  lungs,  arises  as  a  gland-Hke  subdivision  of  the  entodermal  tract. 
Beginning  opposite  the  third  or  fourth  branchial  arch,  two  longitudinal 

grooves  develop,  one  on  either  side  of  the 
embryonic  'pharynx.'  They  deepen  pos- 
teriorly and  unite,  thus  separating  the  ven- 
tral trachea  from  the  dorsal  oesophagus. 
The  trachea  and  oesophagus  open  ante- 
riorly iijto  the  pharynx  of  the  adult.  The 
anterior  end  of  the  trachea,  with  the  epi- 
glottis, thyreoid,  cricoid  and  other  carti- 
lages which  develop  in  the  connective 
tissue  around  it,  constitutes  the  larynx. 
Posteriorly  the  trachea  bifurcates,  as  seen 
in  the  front  view  of  the  embryo.  Fig.  267, 
A,  and  these  primary  subdivisions  or 
bronchi,  further  subdivide  as  shown  in  B. 
In  side  view  the  right  lung  of  an  older  em- 
bryo is  shown  in  Fig.  268;  the  left  lung  has 
been  cut  away.  The  entodermal  outpocket- 
ings  are  seen  to  he  in  abundant  connective  tissue  which  is  invaded  by  blood 
vessels  from  three  sources, — the  pulmonary  arches,  the  left  atrium  and 
the  thoracic  aorta.  Some  branches  which  grow  from  the  azygos  veins  are 
not  shown. 

The  pulmonary  arches  are  two  arteries,  one  on  either  side,  extending 
from  the  ventral  to  the  dorsal  aorta.  Approximately  midway  in  its  course 
each  sends  a  branch  to  the  lung  of  the  corresponding  side.  The  part  of  the 
arch  between  this  branch  and  the  dorsal  aorta  is  early  obUterated  on  the  right 
side,  but  on  the  left  it  persists  until  birth'as  the  ductus  arteriosus  (Fig.  268, 
d.a.).  After  birth  it  is  reduced  to  a  fibrous  cord  which  sometimes  retains 
a  minute  lumen.  The  spiral  division  of  the  ventral  aorta  into  the  proximal 
parts  of  the  permanent  aorta  and  pulmonary  artery,  has  been  referred  to 


Fig.  267.— Reconstructions  of  the 
LvNGS  OF  Young  Embryos,  seen 
FROM  THE  Ventral  Surface. 

A,  A  younger  stage  than  B  :  ep,  apical 
bronchus;  I,  II,  primarv  bronchi. 
(His.) 


RESPIRATORY   TR.\CT. 


235 


in  connection  with  the  heart.  The  pulmonary  artery  of  the  adult  leaves 
the  heart  as  a  subdivision  of  the  ventral  aorta ;  it  diAides  into  right  and  left 
rami,  apparently  simple  vessels,  but  in  reahty  each  of  them  consists  of  the 
proximal  part  of  a  pulmonar}^  arch  together  v*-ith  a  branch  of  that  arch. 
In  Fig.  268,  there  is  no  indication  that  the  left  ramus,  l.r.  includes  a  part  of 
the  left  pulmonary  arch. 

The  pulmonary  veins  grow  out  from  the  left  atrium  as  a  single  vein 
with  four  main  branches.     By  expansion  of  the 
atrium  the  proximal  part  of  the  vein  is  incor-      ,,,,=- 
porated  in  its  wall  and  the  four  branches,  two      ^ 
from  each  lung,  then  open  separately.     The 
capiUary  subdivisions  of  the  veins  anastomose  '^    - 

with  those  of  the  pulmonary   artery   to    form 
the  principal  blood  supply  of  the  lungs. 

The  small  bronchial  arteries  which  supply 
the  connective  tissue  of  the  lungs  are  branches 
of  the  thoracic  aorta,  one  or  two  on  each  side. 
Their  capillaries  join  those  of  the  bronchial 
veins  derived  from  the  azygos  veins.  In  part 
they  connect  with  the  pulmonary  veins. 
Since  the  bronchial  arteries  convey  'arterial 
blood'  w^hereas  the  pulmonary  arteries  contain 
'venous  blood,'  the  former  may  be  compared 
physiologically  with  the  hepatic  artery  in  the 
Kver. 

The  connective  tissue  in  which  the  entoder- 
mal  part  of  the  lungs  ramifies,  occurs  as  a  pair 
of  lateral  sweUings  of  the  mediastinum.  The 
mediastinum  is  the  connective  tissue  surround- 
ing the  oesophagus  and  extending  between  the 
heart  and  the  dorsal  aorta.  It  is  bounded 
on  either  side  by  the  mesothelium  of  the 
body  ca^dty,  and  so  has  the  structure  of  a 
broad  mesenter}'  of  the  heart.  The  pair  of 
mediastinal   sweUings    or  'pulmonar}'   v,-ings' 

project  into  that  portion  of  the  coelom  which  connects  the  median 
pericardial  cavity,  on  either  side  of  the  mediastinum,  with  the 
peritonaeal  cavit}^  These  portions  of  the  coelom  become  cut  off,  first  from 
the  pericardium  and  later  from  the  peritonaeum,  thus  producing  two 
closed  sacs,  the  pleural  cavities.  Each  of  these. is  fined  with  a  continuous 
layer  of  mesothelium,  which,  with  the  underl}dng  connective  tissue,  con- 


kaa 


Fig.  268. — Reconstruction-  of  a 
Part  of  a  Human  Embryo 
of  13.8  MM.  (Dr.  F.  W. 
Thyng.) 

ao.,  Aorta ;  d.  a.,  ductus  arteriosus; 
I.,  entodermal  part  of  the  lung  \ 
I.  at.,  left  atrium;  I.  br.,  left 
bronchus  ;  I.  r.,  left  ramus  of 
pulmonary  artery,  p.  a.;  r.  r., 
its  right  ramus  ;  oe.,  oesopha- 
gus ;  p.  c,  pericardial  cavity ; 
p.  v.,  pulmonary  vein ;  s.  t., 
septum  transversum  ;  th.  ao., 
thoracic  aorta  ;  tr.,  trachea. 


2^6  HISTOLOGY. 


stitutcs  the  pleura.  The  parietal  pleura  is  the  part  attached  to  the  body 
wall;  the  pulmonary  pleura  covers  the  lungs;  other  subdivisions  are  the  medi- 
astinal, pericardial,  and  diaphragmatic  pleurae.  The  lung  is  connected 
with  the  mediastinum  by  a  short  and  broad  stem  of  connective  tissue, 
across  which  the  bronchi,  vessels  and  nerves  extend.  This  is  the  root  of 
the  lung. 

Development  of  the  alveoli.  Fig.  269,  A,  from  an  embryo  of  four 
months,  shows  a  portion  of  the  lung  adjacent  to  the  pleura.  The  terminal 
subdivisions  of  the  bronchi  are  lodged  in  an  abundant,  vascular  connective 
tissue.  They  are  lined  with  a  simple  cuboidal  epithelium  and  are  gland- 
Hke  in  form.  This  appearance  is  retained  until  birth  when  they  become 
distended  wdth  air.  Then  their  cuboidal  cells  are  flattened,  and  many  of 
them  are  transformed  into  thin  non-nucleated  plates  (Fig.  269,  B).     The 


}pi. 


b.v. 


■■^^: 


Fig.  269.— Sections  of  the  \'iscerai.  Pleura,  pi.,  and  Adjacent  Alveoli,  al.,  from  the  Lung  of 
A  Four  Months  Embryo,  A,  and  from  an  Adult,  B. 

ar..  Artery  ;  b.  v..  blood  vessel ;  cap.,  capillary  ;  ly.,  lymphatic  vessel ;  s..  surface  view 
of  alveolar  wall  ;  v.,  vein. 

connective  tissue  between  the  alveoli  is  compressed  into  strands  scarcely 
wider  than  the  diameter  of  a  capillary.  In  fact  the  capillaries  which  they 
contain  are  in  contact  with  the  respiratory  epithelium  of  both  of  the  ad- 
jacent alveoli.  A  section  of  the  adult  lung  is  essentially  a  network  of  these 
slender  partitions,  scattered  among  which  are  islands  of  connective  tissue 
containing  the  bronchi  and  vessels.  There  are  also  connective  tissue 
septa,  dividing  the  lung  into  lobules. 

Summary.  The  lungs  develop  as  a  branched  entodermal  gland  with 
the  trachea  and  bronchi  as  its  ducts.  The  terminal  alveoli  become  greatlv 
distended  and  their  cells  form  fiat  plates  adapted  for  respiration  but  not 
for  secretion.  The  lungs  have  two  sets  of  blood  vessels,  both  capillary 
in  type, — the  pulmonary  and  the  bronchial  vessels.  The  connective  tissue 
forms  a  peripheral  layer  which  is  part  of  the  pleura,  and  a  large  mass  at  the 


LARYNX,  237 

root  of  the  lung.  Within  the  lung  it  forms  interlobular  septa,  and  the  thin 
interalveolar  layers,  but  it  is  most  conspicuous  around  the  bronchi.  In  the 
following  sections  the  structure  of  the  respiratory  tract  \^'ill  be  considered 
beginning  with  the  lar}Tix,  and  proceeding  posteriorly. 

Larynx. 

The  mucous  membrane  of  the  larynx  is  a  continuation  of  that  of  the 
pharynx,  and  likewise  consists  of  an  epithelium  and  tunica  propria.  A 
submucosa  connects  it  with  the  underlying  parts.  In  most  places  the 
epithelium  appears  to  be  stratified  and  columnar,  but  it  is  said  to  be  pseudo- 
stratified,  with  nuclei  at  several  levels.  It  is  difficult  to  determine  whether 
or  not  all  of  the  cells  are  in  contact  with  the  basement  membrane.  This 
type  of  epithelium,  which  occurs  also  in  the  trachea,  is  ciliated.  The 
stroke  of  the  ciHa  is  toward  the  pharynx.  A  stratified  epithehum  with 
squamous,  non-cihated  outer  cells  is  found  on  the  vocal  folds  [true  vocal 
cords],  the  anterior  surface  of  the  arytaenoid  cartilages  and  the  lar}TLgeal 
surface  of  the  epiglottis.  The  distribution  of  the  two  sorts  of  epithelium 
anterior  to  the  vocal  folds  is  subject  to  individual  variation.  The  squamous 
epithelium  often  occurs  in  islands.  The  tunica  propria  consists  of  numer- 
ous elastic  fibers  and  fibrillar  connective  tissue,  which  in  the  lower  animals 
forms  a  dense  membrana  propria  under  the  epithehum.  It  also  includes 
reticular  tissue  containing  a  variable  number  of  leucocytes;  solitarv'  nodules 
may  be  found  in  the  ventricle  of  the  larynx  [sinus  of  Morgagni].  Pap- 
illae in  the  tunica  propria  are  chiefly  in  the  region  of  the  squamous  epi- 
thehum. At  the  free  border  and  on  the  under  surface  of  the  vocal  folds, 
the  papiUae  unite  to  form  longitudinal  ridges.  On  the  laryngeal  surface 
of  the  epiglottis  there  are  only  isolated  papillae,  against  which  rest  the 
short  taste  buds. 

The  submucosa  contains  mixed,  branched,  tubulo-alveolar  glands, 
measuring  from  0.2  to  i.o  mm;  they  are  abundant  in  the  ventricle  but  are 
absent  from  the  middle  part  of  the  free  border  of  the  vocal  folds. 

The  cartilages  of  the  larynx  are  mostly  of  the  hyaline  variety,  resem- 
bling those  of  the  ribs.  To  this  class  belong  the  thyreoid,  cricoid,  the  greater 
part  of  the  arytaenoid,  and  often  the  small  triticeous  cartilages.  Elastic 
cartilage  is  found  in  the  entire  epiglottis,  the  cuneiform  and  comiculate 
cartilages,  the  apex  and  vocal  process  of  the  ar}'taenoids,  and  generally  the 
median  part  of  the  thyreoid.  In  women  this  portion  is  not  involved  in  the 
ossification  Tchiefly  endochondralj  which  begins  in  the  thyreoid  and  cricoid 
cartilages  between  the  twentieth  and  thirtieth  years.  The  triticeous  carti- 
lages (nodules  in  the  lateral  hyothyreoid  hgaments,  named  from  their 
resemblance  to  grains  of  wheat)  are  sometimes  composed  of  fibro-cartilage. 


238  HISTOLOGY. 

The  blood  vessels  form  two  or  three  networks  parallel  with  the  surface, 
followed  by  a  capillary  plexus  just  beneath  the  epithelium.  The  lym- 
phatic vessels  similarly  form  two  communicating  networks,  of  which  the 
more  superficial  consists  of  smaller  vessels  and  is  situated  beneath  the 
capillary  plexus.  The  nerves  form  a  deep  and  a  superficial  plexus  which 
are  associated  with  microscopic  ganglia.  Non-medullated  fibers  end  either 
beneath  the  epithelium  in  bulbs  and  free  endings  with  terminal  knobs,  or 
within  the  epithehum  in  free  ramifications  and  in  taste  buds.  Below  the 
vocal  folds,  subepithehal  nerve  endings  and  buds  are  absent,  but  many 
intraepithelial  fibers  occur  which  encircle  individual  taste  cells.  The 
nerves  and  vessels  of  the  larynx  are  numerous,  except  in  the  dense  elastic 
tissue  of  the  vocal  folds.  The  ventricular  folds  [false  vocal  cords]  consist 
of  loose  fatty,  glandular  tissue  rich  in  vessels. 

Trachea. 

The  trachea  consists  of  a  mucosa,  submucosa,  and  a  fibrous  outer 
layer  containing  the  tracheal  cartilages.  The  outer  layer  is  continuous 
with  the  tissue  of  the  mediastinum.  It  forms  the  perichondrium  sur- 
rounding the  succession  of  hyahne  C-shaped  cartilages,  the  free  ends  of 
which  are  toward  the  oesophagus.  In  the  interval  between  these  ends, 
there  is  a  layer  of  transverse  smooth  muscle  fibers,  usually  accompanied 
by  bundles  of  outer  longitudinal  fibers.  As  in  the  intestine,  elastic  fibers 
are  abundant  among  the  muscle  cells.  The  tracheal  cartilages  may  be- 
come partly  calcified  in  old  age. 

The  submucosa  is  a  layer  of  loose  fatty  connective  tissue,  continuous  on 
its  outer  side  with  the  perichonidrium.  It  contains  the  bodies  of  the 
branched,  mixed  tracheal  glands. '  On  the  dorsal  or  oesophageal  wall  of 
the  trachea,  these  glands  are  larger  than  elsewhere  and  extend  into  or 
through  the  muscle  layers. 

The  mucosa  is  separated  from  the  submucosa  by  a  distinct  dense 
layer  of  elastic  fibers,  chiefly  longitudinal.  This  layer  has  been  com- 
pared with  the  muscularis  mucosae  of  the  intestine.  Between  it  and  the 
epithehum  there  is  a  thin  layer  of  tissue,  containing  elastic  fibers  and 
having  leucocytes  in  its  meshes.  A  basement  membrane  is  found  beneath 
thfe  epithehum.  As  in  the  larynx  the  epithehum  is  pseudo-stratified  and 
columnar,  with  ciha  proceeding  from  distinct  basal  bodies.  It  contains 
goblet  cells.  On  the  oesophageal  surface  there  have  been  found  areas 
of  non-cihated,  stratified  epithelium,  with  connective  tissue  papillae 
beneath,  and  squamous  cells  on  its  surface. 


BRONXHI. 


239 


Broxchi  axd  Broxchioles. 
The  primary  bronchi  have  the  same  structure  as  the  trachea.  In 
their  subdivisions  changes  occur,  the  C  shaped  cartilages  being  replaced 
by  irregular  plates  found  on  all  sides  of  the  tube  (Fig.  270).  These  diminish 
in  size  and  thickness  as  the  branches  of  the  bronchi  become  smaller,  and 
disappear  in  those  about  i  mm.  in  diameter.     Branched  tubulo-alveolar 


Epithelium 


Circular 
muscle  fibers. 


Aheoli. 


^^■^ 


Blood  ^.-Y^-/    / 
vessel.       ^,''.^''  y: 

Fat  cells.  — 'iQ^^i:  I:  : 


Cartila.sre. 


Connective  tissue. 


Bronchial  gland. 


Duct  of  eland. 


Fig.  270. — Cross  Sectio.v  of  a  Bronchus  2  mm.  in  Di.^meter,  fro.m  a  Child. 


glands  occur  as  far  as  the  cartilages  extend.  They  are  situated  in  a  loose 
connective  tissue  layer  containing  many  nerves,  blood  and  lymphatic  ves- 
sels, together  with  small  hmph  glands.  The  bodies  of  the  bronchial  glands 
lie  outside  of  a  rather  loose  smooth  muscle  layer  with  fibers  chiefly  cir- 
ailar.  The  mucosa  is  thrown  into  longitudinal  folds.  It  consists  of  a 
pseudo-stratified  cihated  epithelium  in  the  larger  bronchi,  changing  grad- 
ually to  a  simple  epithehum  in  the  small  ones.  The  stroke  of  the  ciha, 
as  in  the  trachea,  is  toward  the  phar\TLx.     The  epithehum  contains  goblet 


240 


HISTOLOGY. 


cells,  and  rests  on  a  tunica  propria  which  has  many  elastic  tibers  and  lym- 
phocytes.    The  latter  may  accumulate  in  nodules. 

Bronchioles  are  the  small  subdivisions  of  the  bronchi,  measuring 
from  C.5  to  i.o  mm.  in  diameter.  They  are  free  from  cartilage  and  glands 
but  have  a  columnar  cihated  epithelium  throughout.  Obviously  the  dis- 
tinction between  the  smaller  bronchi  and  the  bronchioles  is  arbitrar}^ 
The  terminal  branches  of  the  latter  are  called  respiratory  bronchioles. 

Respiratory    Bronchioles,   Alveolar   Ducts,   Alveolar   Sacs, 

Alveoli. 
An  arrangement  of  the  ultimate  branches  of  a  bronchiole  is  sho^^^l  in 
the  diagram.  Fig.  271.     The  respiratory  bronchioles,  0.5  mm.  or  less  in 
diameter,  at  their  beginning  contain  a  simple  columnar  cihated  epithelium. 


Bronchial  aiterv.  — • 


Pulmonarv  vein. 


—  -»  Pulmonarv  arterv. 


—  - Respiratory  bronchiole. 


Pleural  capillaries 


(Lobule.) 


Fig.  271.— Diagr..\.m  of  \  Lobcle  of  the  Ling,  showing  the  Blood  Vessels  and  the  Tkr.minal 

Br.anches  of  a  Bronchiole. 

Further  in  their  course  the  goblet  cells  disappear,  cilia  are  lost,  the  cells 
become  cuboidal,  and  among  them  are  found  thin,  non-nucleated  plates 
of  different  sizes.  These  plates  together  with  the  isolated  cuboidal  cells 
remaining  among  them  constitute  the  respiratory  epithelium.     The  tran- 


AL\^OLI   OF   THE   LUXG. 


241 


sition  from  the  cuboidal  to  the  respiratory  epitheUum  occurs  u-regu- 
larly,  so  that  a  bronchiole  may  have  cuboidal  epithehum  on  one  side 
and  respirator}'  epithehum  on  the  other;  or  one  sort  of  epithehum 
may  form  an  island  in  the  midst  of  the  other.  Hence  the  respiratory 
bronchioles  contain  a  mixed  epithehum  (Fig.  272,  A).  The  respiratory 
epithehum  steadily  gaias  in  extent  until  the  cuboidal  epithehum  has 
disappeared. 

At  irregular  intervals  along  the  bronchioles  the  respiratory  epithelium 
forms  hemispherical  outpocketings  or  alveoli.  The  alveolar  ducts,  from 
I  to  2  mm.  long,  differ  from  the  respiratory  bronchioles  in  that  they  con- 
tain only  the  respiratory  epithehum  and  are  thickly  beset  with  alveoli. 


Cuboidal 

epithelial        Xon-nucleated 
cells.  plates. 


Pores.       Cuboidal  epithelial  cells.     >^       nu'leated 
V  ^  plates 


1  J 


h       ' 


-^ 


A  Border  of  an  alveolus.      B      Fundus  of  an  alveolus. 

Fig.  272.— Fro.m  Sections  of  the  Human  Lung.     X  240. 
A,  Mixed  epithelium  of  a  respiratory  bronchiole;   B,  an  alveolus  sketched  with  change  of  focus;   the 
border  ot  the  alveolus  is  shaded  ;  it  is  covered  by  the  same  epithelium  as  that  of  the  (clear)  fundus  of 
the  alveolus  ;  the  nuclei  ot  the  cells  are  invisible.     (Silver  nitrate  preparation.) 

The  layer  of  smooth  muscle  fibers  may  be  traced  to  the  end  of  the  alveolar 
ducts,  where  it  terminates.  Since  the  muscles  do  not  extend  over  the 
alveoh,  but  merely  surround  the  main  shaft  of  the  duct,  the  layer  is  greatly 
uiterrupted,  and  some  consider  that  it  ends  in  the  course  of  the  duct. 
The  respiratory  bronchiole  may  be  continued  as  a  single  alveolar  duct  or 
may  divide  into  two  or  more. 

The  alveolar  ducts  branch  to  produce  alveolar  sacs  [infundibula] 
which  are  cavities  in  the  center  of  clusters  of  alveoh.  The  sacs  resemble 
the  ducts  as  shovm  in  Fig.  271.  According  to  Professor  Miller,  who  has 
made  reconstructions  of  these  structures  in  the  human  lung,  an  atrium  or 
round  cavity  should  be  recognized  between  the  alveolar  duct  and  the  alveo- 
lar sacs.  The  alveolarrduct  opens  sometimes  into  five  atria  from  each  of 
16 


242 


HISTOLOGY. 


Fig.  273.  —  Camera  Lucida  Drawing  from  a 
Section  of  a  Calf's  Lung.     (Miller.) 

The  stippling  indicates  smooth  muscle  and  cuboid- 
al  epithelium ;  the  lines,  respiratory  epithe- 
lium. B.  R.,  Respiratory  bronchiole  ;'D.  A.,  al- 
veolar duct ;  A.,  atrium  ;  A.  S.,  alveolar  sac. 


which  several  ah-eolar  sacs  proceed  (Fig.  273).  If  the  student  in  examining 
this  figure  questions  why  the  atria  are  not  areolar  ducts,  and  the  ah'eolar 
ducts  are  not  respiratory  bronchioles,  it  may  be  said  that  these  terms  are 
variously  employed  by  different  histologists,  and  that  atria  are  not  recog- 
nized by  German  writers.  It  seems  questionable  that  the  final  ramifi- 
cations of  the  lung  are  so  definitely 
arranged  as  to  justify  the  cumber- 
some nomenclature  in  current  use. 
Fig.  273  shows,  however,  exactly 
what  may  be  expected  in  any  sec- 
tion of  the  lung,  namely  (i)  alveoli; 
(2)  spaces  bounded  by  alveoli 
(alveolar  sacs,  atria,  alveolar  ducts, 
the  last  being  supposed  to  have  mus- 
cle fibers  associated  with  them) ;  (3) 
small  bronchioles  with  alveoli  along 
their  walls,  therefore  consisting  of 
a  mixed  epithelium  (respiratory 
bronchioles);  and  (4)  bronchioles 
with  no  respiratory  epithehum. 

The  alveolar  walls  have  been 
described  as  consisting  of  respiratory  epithehum"^  (Fig.  272,  B).  The 
non-nucleated  plates  are  presumably  derived  from  the  flattened  nucleated 
cells  scattered  among  them,  and  large  plates  arise  from  the  fusion  of  small 
ones.  In  amphibia,  nuclei  in  small  amounts";of  ^protoplasm  are  found 
attached  to  the  edges  of  the  plates,  and  projecting  into  the  connective 
tissue  between  the  capillaries. 
The  abundant  capillary  network 
of  the  alveolar  walls  is  sho^^^l  in 
Fig.  274;  lymphatic  vessels  are 
absent.  Elastic  tissue  is  highly 
developed  around  the  alveoli  and 
forms  rings  encircHng  their  out- 
lets. In  inspiration  an  alveolus 
may  expand  to  three  times  the 
diameter  to  which  it  returns  dur- 
ing expiration  (o.i  to  0.3  mm.). 

Pores  have  been  described,  leading  from  one  alveolus  to  another  (Fig.  272, 
B). 

The  pleura  is  essentially  similar  to  the  peritonaeum,  consisting  of  a 
connective  tissue  layer  covered  with  a  flat  epithelium  (mesothelium).     Pcr- 


Vein. 


—  Capillaries 


Arten 


Fig.  274.  —  From  a   Section  of  the   Lung  of  a 
Child,    Injected    through    the    Pulmonary 
Artery.    X  80. 
Of  the  five  alveoli  drawn  the  three  upper  ones  are 
fully  injected. 


PLEURA.  243 

manent  apertures  (stomata)  in  the  epithelium  probably  do  not  exist.  The 
connective  tissue  of  the  pulmonary  pleura  contains  many  elastic  fibers; 
these  are  less  abundant  in  the  parietal  pleura.  Fat  is  found,  sometimes 
forming  folds  {plicae  adiposae)  and  the  vascular  elevations  suggestive  of 
synovial  villi  are  called  pleural  villi.  These  may  be  sought  toward  the 
median  wall,  beneath  the  lung.  The  nerves  of  the  pleura,  derived  from 
the  phrenic,  sympathetic  and  vagus  are  said  to  possess  small  gangha.  In 
the  parietal  pleura  t3^ical  lamellar  corpuscles  and  some  of  their  varieties 
(Golgi-Mazzoni  corpuscles)  have  been  found.  The  blood  vessels  of  the 
pleura  are  said  to  include  branches  both  of  the  pulmonary  and  the  bron- 
chial vessels.  Lymphatic  vessels  are  numerous  and  small  lymph  glands 
occur. 

Septa  extend  from  the  pleura  into  the  lung  thus  dividing  its  super- 
ficial portion  into  lobules  from  i  to  3  cms.  in  diameter.  They  are  visible 
on  the  surface  as  polygonal  areas  bounded  by  pigmented  Hnes.  Since 
these  lobules  consist  of  smaller  subdivisions  also  called  lobules,  the  former 
are  designated  as  secondary  and  the  latter  as  primary  lobules  {structural 
units) . 

In  the  connective  tissue  between  the  secondary  or  larger  lobules, 
lymphatic  vessels  make  their  way  to  the  pleura  and  thence  over  the  surface 
of  the  lung  to  its  root.  These  lymphatic  vessels  constitute  the  superficial 
system.  The  deep  lymphatic  vessels  begin  along  the  small  bronchioles 
and  the  adjoining  vessels,  and  they  accompany  the  arteries,  veins,  and 
bronchi  to  the  root  of  the  lung.  To  some  extent  the  superficial  and  deep 
systems  communicate.  No  lymphatic  vessels  are  found  beyond  the  alveo- 
lar ducts,  within  the  lobules.  Along  the  larger  bronchi  and  toward  the 
root  of  the  lung  lymph  glands  are  numerous. 

Black  pigment  is  generally  abundant  along  the  course  of  the  lymphatic 
vessels.  It  is  not  melanin  but  soot,  which  is  absent  from  the  lungs  at  birth 
but  accumulates  with  age,  especially  in  certain  environments.  It  pene- 
trates the  pulmonary  epithehum  chiefly  in  the  smallest  bronchioles,  ap- 
parently passing  between  the  cells.  Some  of  it  is  taken  up  by  phagocytes. 
Having  entered  the  lymphatic  vessels  it  becomes  distributed  along  their 
courses. 

The  blood  vessels  accompany  the  bronchi.  In  the  primary  or  ultimate 
lobules  the  arteries  are  central,  producing  a  termj'nal  branch  for  each 
atrium  or  alveolar  sac  (Fig.  271).  The  veins  arising  from  the  alveolar 
capillaries  pass  over  the  peripheral  surface  of  the  structural  units  as  shown 
in  the  figure.  The  distribution  of  the  bronchial  vessels  has  already  been 
noted. 

The  nerves  of  the  lung  include  a  pulmonary  plexus  from  the  sympa- 


244 


HISTOLOGY. 


thetic  system,  which,  entering  at  the  root,  accompanies  the  bronchi  and 
vessels;  to  them  it  is  chiefly  distributed.  Small  gangha  are  found  within 
it.  The  vagus  also  sends  important  branches  to  the  lung,  which  mingle 
with  the  perivascular  and  peribronchial  nerves.  They  contain  both 
medullated  and  non-medu Hated  fibers. 


URINARY   ORGANS. 
Wolffian  Body. 

The  Wolffian  body  or  mesonephros  is  the  "kidney"  of  adult  amphibia 
and  of  certain  fishes.  It  is  one  of  the  largest  organs  found  in  the  human 
embryo  of  the  second  month,  but  subsequently 
its  renal  functions  are  performed  by  another 
structure  of  later  development, — the  kidney  {meta- 
nephros).  As  the  Wolffian  body  degenerates  it 
becomes  transformed  in  the  male  into  the  ductus 
deferens  and  the  epididymis,  essential  portions  of 
the  genital  tract.  Some  vestigial  remnants 
may  produce  pathological  growths.  In  the 
female  the  entire  organ  is  vestigial,  with  patho- 
logical possibihties.  During  its  development  and 
regression  the  Wolffian  body  is  a  controlling  fac- 
tor in  the  arrangement  of  the  large  veins  of  the 
abdomen. 

In  an  embryo  of  35  days  (Fig.  275)  the  Wolff- 
ian bodies  are  seen  as  a  pair  of  long,  rounded 
elevations,  one  on  either  side  of  the  root  of  the 
mesentery.  They  extend  the  length  of  the  ab- 
dominal cavity  and  each  empties  through  its  Wolff- 
ian duct  into  the  allantois  (described  on  p.  193). 
The  excretion  of  the  Wolffian  bodies  accumulates  in  the  allantois,  which  in 
man  is  a  slender  but  very  long  tube.  In  the  pig  at  a  certain  stage,  it  is 
an  elongated,  thin- walled  sac  many  times  the  size  of  the  entire  embryo; 
the  large  amount  of  fluid  which  it  contains  is  due  to  an  unusual  develop- 
ment of  the  Wolffian  bodies.  After  the  urogenital  sinus  opens  to  the 
exterior,  the  contents  of  the  allantois  may  mingle  with  the  amniotic  fluid 
in  which  the  embryo  is  immersed. 

Development  of  the  Wolffian  body.  In  a  previous  section  (p.  22) 
the  development  of  the  mesoderm  has  been  described  to  that  stage  when 
it  presents  a  series  of  segments  (protovertebrse),  connected  by  stalks 
(nephrotomes)  with  the  layers  which  fine  the  body  cavity.     From  several 


Fig.  275. — Dissection  of  a 
Human  Embryo  of  35 
Days.    (After  Coste.) 

al.,  Allantois  ;  I.,  lung;  st.,  stom- 
ach ;  s.tr.,  septum  transver- 
sum  ;  u.  c,  umbilical  cord  ; 
W.b.,  Wolffian  body;  W.  d., 

Wolffian  duct. 


WOLFFIAN    BODY. 


24 


-fD 


of  the  anterior  nephrotomes  there  arise  rounded  elevations  which  grow 
posteriorly  and  unite  with  one  another  to  form  a  longitudinal  cord  of  cells 
on  either  side  of  the  body.  This  later  becomes  hollow  and  is  kno^Ti  as 
the  Wolffian  duct.  In  a  rabbit  embryo  it  is  shown  in  Fig,  276,  A.  As  the 
Wolffian  duct  extends  posteriorly  it  lies  so  close  to  the  ectoderm  that  the 
latter  has  been  said  to  participate  in  its  formation.  Finally  it  reaches  and 
fuses  with  the  entodermal  allantois.  The  posterior  nephrotomes  are  not 
thought  to  contribute  to  the  formation  of  the  duct.  As  seen  in  Fig.  276,  B, 
they  become  separated  both  from  the  segments  (iny)  and  the  coelomic 


mes.seg". 


Wd. 


Fig.  276. — A,  Transverse  Section  of  a  Rabbit  Embryo  of  Nine  Days;  B,  Human  Embryo,  4  mm.; 

C,  Human  Embryo,  10  mm. 

ao,  Aorta;  c,  posterior  cardinal  vein;  coe.,  coelom  ;  al.,  glomerulus  ;  g.  r.,  genital  ridge  ;  int.,  intestine  ; 
mes.,  mesentery ;  mes.  seg.,  mesodermic  segment  ;  my.,  myotome  ;  nch.,  notochord;  neph..nephrotome  ; 
S-C.  v.,  subcardinal  vein  ;  si.,  sinusoid  ;  sy.,  svmpathetic  nerves  ;  u.  v.,  umbilical  vein  ;  W.  d.,  Wolffian 
duct ;  W.  t.,  Wolffian  tubule. 


epithehum.  The  nephrotomes  form  vesicles  (TF.^)  which  become  tubular 
and  coiled;  each  acquires  connection  with  the  Wolffian  duct  (Fig.  276,  C), 
By  branching  or  fission  the  tubules  become  more  numerous  than  the  cor- 
responding segments. 

The  aorta  sends  a  succession  of  branches  to  the  ventro-median  border 
of  the  Wolffian  body.  There  they  terminate  in  round  knots  of  capillaries 
known  as  glomeruli  (Fig.  276,  C).  A  glomerulus  is  at  iirst  lodged  in  a 
cup  shaped  depression  on  one  side  of  a  Wolffian  tubule,  at  its  bhnd  end. 
The  tubule  then  grows  around  the  glomerulus  so  that  the  latter  appears 


246 


HISTOLOGY. 


invaginated  into  its  globular  distal  extremity  (Fig.  277).  The  tubule  is 
said  to  form  the  capsule  of  the  glomerulus,  consisting  of  an  outer  and  an 
inner  layer  between  which  is  an  extension  of  the  lumen  of  the  tubule. 
The  layers  are  continuous  with  one  another  at  the  stalk  of  the  glomerulus. 
There  the  efferent  vessel  may  be  found  near  the  afferent  artery  as  in  the 
figure,  or,  as  has  been  described  in  the  pig,  several  radiating  efferent  ves- 
sels may  leave  the  capsule  at  different  points.  Whether  these  all  emerge 
through  one  crescentic  aperture  in  the  capsule,  or  whether,  by  coalescence 
of  its  edges  between  the  vessels,  they  leave  through  separate  openings,  has 
not  been  determined.  The  stalk  and  its  tubule  may  both  be  on  one  side  of 
the  capsule,  and  not  at  its  opposite  poles  as  in  the  figure.     From  the  blood 

circulating  through  the  glomer- 
ulus, fluid  "filters"  into  the  tu- 
bule, forming  the  greater  part  of 
the  urine. 

The  tubules,  starting  from 
the  ventro-medial  glomeruli,  fol- 
low a  convoluted  course  to  the 
Wolffian  duct.  In  the  pig  two 
tubules  have  been  found  to  unite 
before  entering  the  duct,  and 
near  the  glomeruli  they  may  fork 
FIG.  377.-REcoNSTRucT,oN  OP  A  wcPKiAN  TuBULH    SO  as  to  conncct  with  two  cap" 

t^llo.'^neruVutaftfrKXa^n^)"-^""-      ^^''''''"     SUlcS.       A    bhnd     divcrticulum    is 

C,  Inner  layer,  and  c.  a.,  outer  layer  of  the  capsule  of     shown  in  Fig.   277.       Thc  tubuleS 
the  glomerulus;    div.,    diverticulum;    gl.,    glomer-  t         1        i  1  •  1         • 

uius ;  w.  d.,  Wolffian  duct.  are    hncd    thoughout    with  sim- 

ple epitheKum.  It  is  flat  in  the 
capsule  where,  in  the  pig,  it  is  said  to  be  thinner  in  the  outer  layer; 
the  reverse  condition  has  been  figured  for  the  human  embryo.  The 
remainder  of  the  tubule  may  be  divided  into  conducting  and  secretory 
portions.  The  latter,  found  in  the  middle  part  of  the  tubule,  has  low 
columnar  epithehum  with  dark  basal  protoplasm  and  a  clear  vacuolated 
appearance  toward  the  lumen.  These  cells  are  supposed  to  excrete  a 
portion  of  the  urine.  The  conducting  tubules  have  a  cuboidal  epithehum 
without  indications  of  glandular  activity.  The  secreting  and  conducting 
portions  of  the  kidney  tubules  have  been  more  thoroughly  studied  than 
those  of  the  W^olffian  tubules. 

Veins  0}  the  Wolffian  body.  Early  in  embryonic  life  two  vessels  arise 
from  the  vitelline  veins  close  to  their  entrance  into  the  atrium  and  grow 
forward  into  the  head,  one  on  either  side.  These  are  the  anterior  cardinal 
veins,  and  from  each  of  them  a  posterior  cardinal  vein  growls  along  the 


MEIXS    OF   THE    WOLFZI.AA'   BODY. 


247 


aorta  toAvard  and  into  the  tail.  (Veins  and  arteries  in  its  path  contribute 
to  its  formation.)  Duct  of  Cuvier  is  the  name  of  the  single  vessel  on  each 
side  which  conveys  the  blood  from  the  cardinal  veins  to  the  right  atrium; 
the  left  duct  of  Cuvier  crosses  the  dorsal  surface  of  the  heart  in  the  atrio- 
ventricular groove.  The  early  arrangement  of  the  cardinal  veins  is  shown 
in  Fig.  278,  A.  A  Wolffian  body  has  developed  in  the  path  of  each  pos- 
terior cardinal  vein,  and  has  been  a  factor  in  causing  the  vein  to  form  the 
elongated  loop  shoA\Ti  in  the  figure.     The  dorso-lateral  limb  of  the  loop 


Fig.  27S.- 


-The  Transformatiox  of  the  Posterior  Cardinal  Veins  of   Man,  C  Representing 
THE  Adult.    The  Wolffian  Body  is  Dotted. 


a.  C,  anterior  cardinal ;  as.  I.,  ascending  lumbar;  az.,az3'gos;  c,  caudal ;  c.  C,  cisterna  chyli ;  c.h.,  com 
mon  hepatic;  c.  il.,  common  iliac;  c.  S.,  coronary  sinus  ;  d.  C.,  duct  of  Cuvier;  g.,  spermatic  or  ova- 
rian ;  h.,  hiepatic;  h-az.,  hemiazygos  ;  h-az.ac,  accessory  hemazygos  ;  i.  j.,  internal  jugular;  I.  c.  i. 
left  common  iliac;  1.  in.,  left  innominate;  m.  s.,  median  sacral;  p.  c,  posterior  cardinal;  r.,  renal; 
r.  a.,  renal  anastomosis;  r.  C.  i.,  right  common  iliac;  r.  in.,  right  innominate:  s.,  suprarenal ;  S-C, 
subcardinal ;  s-ci.,  subclavian  ;  si.,  sinusoids  ;  v.  C.  i.,  vena  cava  inferior ;  v.  C.  S.,  vena  cava  superior. 


is  the  main  stem  of  the  posterior  cardinal  vein;  it  receives  the  intersegmental 
veins  (lumbar  and  intercostal).  The  ventro-medial  hmb  of  the  loop  is  the 
subcardinal  vein  found  near  the  root  of  the  mesenter}^,  as  seen  in  the  cross 
section,  Fig.  276,  C.  Sinusoids  extending  among  the  Wolffian  tubules 
connect  the  cardinal  and  subcardinal  hmbs  with  one  another.  (They 
are  shown  only  on  the  right  of  Fig.  278,  A.)  The  sinusoids  are  less  nu- 
merous in  mammals  than  in  selachians  and  reptiles. 

The  hepatic  veins  (Fig.  278,  A)  are  ventral  to  the  subcardinals,  which 
are  at  the  root  of  the  mesentery.     When,  however,  the  right  lobe  of  the 


248  HISTOLOGY. 

liver  fuses  with  the  dorsal  body  wall  making  the  coronary  ligament,  the 
right  subcardinal  connects  with  the  hepatic  system,  as  sho\\Ti  in  Fig.  278,  B, 
thus  making  the  inferior  vena  cava.  The  vena  cava  consists  of  the  right 
subcardinal  vein  from  the  liver  to  an  anastomosis  between  the  two  sub- 
cardinals,  known  as  the  renal  anastomosis;  beyond  this  point  it  is  continued 
through  Wolfhan  sinusoids  into  a  portion  of  the  posterior  cardinal.  The 
part  of  the  subcardinals  distal  to  the  anastomosis  is  apparently  the  source  of 
the  cistema  chyh,  and  the  associated  lymphatic  vessels  (Fig.  161,  p.  138). 

With  the  formation  of  the  vena  cava  and  the  regression  of  the  Wolffian 
body,  the  network  of  Wolffian  sinusoids  becomes  separated  from  the  veins 
which  entered  it  posteriorly,  and  from  those  which  drained  it  anteriorly. 
From  the  network  one  large  vein  is  differentiated  (derived  in  part  from 
the  posterior  cardinal)  called  the  spermatic  or  ovarian  vein  according  to 
sex;  the  remnants  of  the  sinusoids  are  tributaries  of  this  vein.  The  kidneys 
come  to  lie  opposite  the  renal  anastomosis,  from  which  the  renal  veins  grow 
out  to  enter  them.  The  reduction  of  the  posterior  cardinal  veins  to  form 
the  azygos  system  of  the  adult,  and  the  formation  of  the  superior  vena  cava 
from  the  anterior  cardinals  are  sho\ATi  in  Fig.  278. 

The  arteries  0}  the  Woljfian  body  are  a  series  of  branches  of  the  aorta, 
each  of  which  supphes  one  or  more  glomeruh.  They  pass  between  the 
posterior  cardinal  and  the  subcardinal  veins  as  seen  in  Fig.  276,  C.  The 
vessels  formed  by  the  union  of  the  capillaries  of  a  glomerulus  empty  into 
the  Wolffian  sinusoids.  With  the  regression  of  the  mesonephros  one  of 
these  arteries, — the  future  spermatic  or  ovarian — sends  branches  into  the 
neighboring  genital  gland  (Fig.  276,  C,  g.  r.).  There  it  unites  with  veins 
which  grow  in  from  the  Wolffian  sinusoids  to  make  a  capillary  circulation. 

Pronephros. 

Anterior  to  the  Wolffian  body  there  occurs,  in  the  lower  vertebrates 
especially,  another  renal  organ  known  as  the  pronephros.  Its  development 
precedes  that  of  the  Wolffian  body.  The  pronephric  tubules  are  segmental 
structures  derived  from  the  nephrotomes  and  characterized  by  retaining 
their  connection  with  the  coelom  and  by  having  their  glomerulus  {glomus) 
on  the  side  of  the  tubule  instead  of  at  the  end.  Since  the  Wolffian  duct  is 
considered  to  be  primarily  the  duct  of  the  pronephros  it  is  often  cahed  the 
pronephric  duct;  the  Wolffian  tubules  become  connected  with  it  second- 
arily. 

In  mammals  the  pronephros  is  scarcely  distinguishable.  Its  tubules 
are  said  to  begin  with  the  4th  or  5th  segment  and  to  extend  to  the  9th  in 
sheep  or  the  nth  in  rabbits.  They  are  transient  structures  imperfectly 
formed.     In  human  embryos  of  3  to  5  mm.  one  or  two  rudimentary  pro- 


DEVELOPMEXT    OF    THE    KIDXEY. 


249 


nephric  tubules  have  been  described.  In  one  case  a  detached  portion  of 
the  Wolffian  duct  opposite  the  6th,  7th  and  8th  segments  has  been  thought 
to  be  associated  with  the  pronephros. 

KiDXEY. 

The  kidney  develops  after  the  Wolffian  body  has  been  formed.  It 
arises  in  two  parts,  one  an  outgrowth  of  the  Wolffian  duct;  and  the  other, 
a'mass  of  dense  mesenchyma  which  is  said  to  be  derived  from  the  posterior 
nephro tomes.  In  this  mesenchyma  tubules  are  formed,  which  have  at 
one  end  glomeruh  similar  to  those  of  the  Wolffian  body,  but  smaller.  The 
tubules  follow  a  contorted  course  and  acquire  their  openings  into  the 
outgrowth  of  the  Wolffian  duct.  The  kidney  is  a  more  complex  organ  than 
the  Wolffian  body,  yet  it  is  constructed  on  a  similar  plan. 


Wd.    M.d.  M.d. 
w.d. 


Fig.  279. — The  Development  of  the  Renal  Pelvis  and  Ureter.     (After  Keibel.) 
A,  Human  embr}©  of  11.5  mm.  (4}^  weeks);  B,  25  mm.  (8^^-g  weeks),     a.,  Anus;  al.  d.^  allantoic  duct; 
bl.,  bladder ;  cl.,  cloaca;  M.  d.,  Miillerian  duct ;  r.,  rectum  ;  ur.,  ureter;  u.  S.,  urogenital  sinus;  W.d., 
Wolffian  duct. 

Development.  An  outpocketing  of  each  Wolffian  duct  near  its  en- 
trance into  the  allantois  becomes  elongated  and  dilated  at  its  distal  end 
(Fig.  279,  A).  The  tubular  part  becomes  the  ureter  and  the  lobed  terminal 
expansion  is  the  renal  pelvis.  As  the  allantois  expands  to  become  the 
bladder,  a  portion  of  the  Wolffian  duct  is  taken  up  into  its  wall  so  that  the 
ureters  acquire  orifices  independent  of  the  Wolffian  ducts;  the  latter  are 
carried  toward  the  median  hne  and  the  outlet  of  the  bladder,  as  shown 
in  Fig.  279,  B.  The  figure  shows  their  permanent  relation  to  the 
ureters. 

In  later  stages  the  lobes  of  the  renal  pehis  become  deeper  and  form 
the  major  and  minor  calyces.  In  the  adult  there  are  usually  two  major 
calyces,  one  at  either  end  of  the  pehds,  and  from  these  most  of  the  minor 
calyces  grow  out;  the  others  spring  directly  from  the  main  pelvic  ca^dty. 
There  are  about  eight  in  all.     From  the  minor  calyces  the  collecting  tubules 


25o 


HISTOLOGY, 


grow  out.     Each  tubule  has  an  enlarged  extremity  (Fig.  280)  which  divides 
into   two  branches  with   a    U-shaped   crotch,   like   a  tuning-fork.     The 


CORTEX 
_  --Pars  Radiata 
Pars  Convoluta 


—  Pyramid 

1.V"  °  °-°  °  •  \       (Medulla) 

%1  7,"^  ^/-Papilla 
r.y^  Renal  Column 


■^as:'!c;^sg??^Ss« 


S^^^^^^^^>.- 


Fig.     280.  —  Reconstruc-  p,g    281— Cross  Section  ok  an  Adult  Kidney. 

TION    OF  THE    LRETKR.  ,„,.   ,  ...  .  ^  „..,,, 

Renal  Pelvis,  and  its  (With  modifications,  after  Brodel.) 

Branches  in  a  20  mm. 
Human  Embryo.  (Hu- 
ber,  Amer.  Journal  of 
Anat.,  Suppl.  to  vol.  iv.) 

branches  subdivide  repeatedly  in  the  same  manner,  so  as  to  make  pyram- 
idal masses  of  straight  tubules  radiating  from  the  calyces.  From  2  to  9 
primary  pyramids  are  said  to  fuse  to  form  a  macroscopic  pyramid  of  the 

adult  kidney  (Fig.  281).  The 
nipple-like  apex  of  the  pyramid 
projects  into  the  renal  calyx  form- 
ing a  renal  papilla.  Each  pa- 
pilla is  covered  by  the  pelvic  epi- 
thehum,  which  is  continuous 
with  that  which  hnes  the  collect- 
ing tubules.  The  trunks  of  these 
tubules  near  the  papilla  are  called 
papillary  duds  and  their  outlets 
are  named  foramina.  Each  pa- 
pilla has  from  15  to  20  foramina. 
Sometimes  two  papillae  project 
into  one  calyx. 

The  renal  pyramids  consti- 
tute the  medulla  of  the  kidney. 
Except  toward  their  apices  they  are  surrounded  by  cortical  substance.     The 
cortex  forms  the  peripheral  part  of  the  kidney,  and  it  also  dips  down  be- 
tween the  pyramids  almost  to  the  pelvis.     In  this  way  the  cortex  forms 


t^>' 


c- 

b 


a 

Fig.  2S2.— From  a  Section  oe  a  Kidney  of  an  i8 
MM.  Human  Embryo.  X  233.  (Huber,  Amer. 
Journal  of  Anat.,  Suppl.  to  vol.  iv.) 

a.,  Primary  coUectiiii;  tubule,  with  dilated  extremity; 
b,  b'.,  inner  layer,  and  c,  outer  layer  of  dense  mes- 
cnchyma  ;  d.,  loose  niesenchyma;  e.,  vesicle,  the 
beginning  of  a  renal  tubule. 


DEVELOPMENT   OP   THE   KIDXEY 


Fig.  2S3.-A  Series 

EROUS   TUBU 

From  a  human  embryo'  of  ^he 


'--^o.rrv%l^i,  Ku^Ll"?m  S-CEssn.H-ST.cHs  I.  xhI 

^  .  CZ.UDING   THK   ASSOCUTHD    PoRTION   OH   THE    CoL-iTeCt/nS^TuBUi"    '"'"'"" 


^E  Development  of  a  Ui 


seventh  month,     v  160      (l^uh.r-A       7  ^'^^"'=-'-'^«ti   1  ubule. 

X  160.     (Huber,  Am.  Jour,  of  Anat.,  Suppl.  to  vol.  iv.) 

almost  to  its  surface      Th 


252 


HISTOLOGY. 


has  the  following  history.  It  becomes  subdivided  into  masses  enveloping 
the  enlarged  tips  of  the  branching  collecting  tubules.  Some  of  its  cells 
become  arranged  so  as  to  form  vesicles  as  sho\ATi  in  the  section  Fig.  282, 
and  in  the  reconstruction  Fig.  283,  A.  In  these  the  vesicle  is  independent 
of  the  collecting  tubule.     In  B  and  C  it  has  become  elongated  making  an 

S-shaped  tubule,  and  has  united  with  the  col- 
lecting tubule.  A  glomerulus  develops  in  the 
lower  curve  of  the  S  and,  as  shown  in  the  figures, 
it  gradually  becomes  enclosed  in  its  capsule — 
the  terminal  part  of  the  tubule.  The  glomer- 
uli begin  to  form  near  the  surface  of  the  kidney 
and  become  buried  in  the  advancing  cortex;  the 
oldest  glomeruli  are  nearest  the  medulla. 

Between  the  capsule  and  the  collecting 
tubule,  the  tubule  of  mesenchymal  origin  be- 
comes contorted  or  convoluted.  One  of  the 
loops  in  the  midst  of  the  coil  elongates  down- 
ward toward  the  medulla,  lying  close  beside 
and  parallel  with  the  collecting  tubules.  This 
Henle''s  loop  (shown  only  in  J  of  Fig.  283)  is 
lodged  in  the  radiate  part  of  the  cortex  and  ex- 
tends into  the  medulla. 

Three  tubules  of  the  adult,  with  capsules 
situated  in  the  outer,  middle,  and  inner  part  of 
the  cortex  respectively,  are  shown  in  the  diagram 
Fig.  284.  Each  capsule  connects  with  a  proxi- 
mal convoluted  tubule  which  is  continuous  with 
the  descending  limb  of  Henle's  loop,  after  hav- 
ing, extended  toward  the  surface  of  the  kidney 
in  the  convolute  part  of  the  cortex.  The  de- 
scending Hmb  is  essentially  a  straight  tube  of 
small  diameter,  owning  to  the  flatness  of  its  cells 
and  not  to  a  narrowing  of  the  lumen.  The 
portion  of  the  proximal  convoluted  tubule 
wdiich  descends  in  a  straight  course  to  join 
the  descending  limb  is  called  the  'end  segment'  or  'spiral  tubule.'  The 
descending  limb  generally  becomes  of  large  diameter  before  it  turns  to 
become  the  ascending  limb  of  Henle's  loop.  This  returns  to  the  immediate 
neighborhood  of  its  capsule,  where  it  forms  the  distal  convoluted  tubule 
[intercalated  tubule].  By  means  of  the  'junctional'  or  'arched  collecting 
tubule'  the  distal  convoluted  joins  the  straight  collecting  tubule.     The 


Fig.  2S4.  — Diagram  of  Three 
Uriniferous  Tubules  and 
THEIR  Relation  to  a  Col- 
lecting Tubule.    (Huber.) 

a.  I.,  Ascending  limb  of  Henle's 
loop;  c,  capsule  ;  C.  t.,  collect- 
ing tubule;  d.  C,  distal  convo- 
luted tubule;  d.  I.,  descending 
limb;  p.  c.,  proximal  convo- 
luted tubule;  p.  d.,  papillarv 
duct. 


KIDNEY. 


'■^6 


uriniferous  tubule  has  no  branches  between  the  capsule  and  the  collecting 
tubule,  but  there  are  many  branches  connected  with  the  latter,  as  shown 
in  the  figure.  The  rounded  "tuning  fork"  crotches  have  become  angular. 
The  straight  tubules,  including  Henle's  loops  and  the  collecting  tubules, 
constitute  the  medulla  and  radiate  part  of  the  cortex.  The  remainder  of 
the  cortex  (pars  convoluta)  [labyrinth]  contains  the  capsules  together  with 
proximal  and  distal  convoluted  tubules  and  arched  collecting  tubules. 

Renal  corpuscle.         Convoluted  tubules.        Pars  radiata. 

i  I 

!  I  \ 

I  ^^..,rr-ry;^~'-.'(''''"^^'-T^''^T^'^^       ' "    -     --  Interlobular  vein. 

,-.'■.■  f 
>.>■-'■■ 


^r- 


Henle's  loop.  Arciform  vein.        Arciforra  artery. 

Fig.  285.— Part  of  a  Radial  Section  of  a  Human  Kidney.    X  25. 
At  X  a  renal  corpuscle  has  dropped  out. 

Smce  a  radial  section  of  the  kidney  shows  both  the  cortex  and  the 
medulla,  it  is  the  kind  made  for  pathological  exanunation.  Under  low 
magnification  such  a  section  is  show^n  in  Fig.  285.  The  renal  corpuscles 
[Malpighian  corpuscles]  are  the  glomeruH  together  with  their  capsules. 
With  higher  magnification  the  various  tubules  of  the  radiate  portion  may 


254  HISTOLOGY. 

be  identified  (Fig.  286) ;  they  may  be  studied  to  better  advantage,  however, 
in  tangential  sections  of  the  kidney,  one  through  the  cortex  and  one  through 
the  medulla.  In  these  the  tubules  appear  in  cross  section.  The  radiate 
parts  of  the  cortex  are  seen  as  islands  of  circular  sections  surrounded  by 
the  irregular  convoluted  tubules  and  renal  corjDuscles.  The  greater  part 
of  such  an  island  is  shown  in  Fig.  287. 

Finer  structure  of  the  renal  tubules.  The  renal  tubules  are  lined 
throughout  with  simple  epithehum.  In  the  inner  layer  of  the  capsule  of 
the  glomerulus,  it  is  a  flat  syncytial  layer  blending  with  the  small  amount 
of  perivascular  connective  tissue  beneath.  The  outer  layer  of  the  capsule 
is  also  flat  and  is  composed  of  polygonal  cells.     Terminal  bars  which  occur 

in  all  other  divisions   of  the  renal 
r  tubules  have  not  been  demonstrated 

(j-j  .£,  ...  in  the  capsule.     The  flat  epithehum 

Vc:  <j'  of  the  outer  layer  of  the   capsule 

-'' '   -  -^T  Collecting  tubule,     changes  at  the  'neck'  of  the  capsule 

to  the  low  columnar  epithelium  of 

Descending  limb.  ,  .         .  i  i  i      i 

the    proximal    convoluted    tubule. 
Here  cell  boundaries  are  indistinct. 

^—.Ascending  limb.  ^j^^   ^^^^j^-    ^^^   ^^^^^^     ^^^    ^^^^  ^j 

the  cells  which  rest  on  a  structure- 
-  Surface  ^■ie^v  of       Icss  bascmcnt  membrane  continuous 

an  ascending 

limb.  with  that  of  the  capsule.     The  pro- 

l  ■       jy  toplasm  contains  granules  arranged 

^         *'  in  vertical  rows  which  tow^ard   the 

base  of  the  cell  appear  as  rods  (Fig. 

280).     In  certain  animals  plaitings 

Fig.  286.— Tubules  of  the  P.ars  Radiat.^.  _     "^^  i  o 

From  a  radial  section  of  a  human  kidney.     X  240.        in  the  CCll  Wall  have    bcCn    foUnd    tO 

cause  a  rodded  appearance  in  these 
cells.  Toward  the  irregular  lumen  there  is  a  'brush  border'  (Fig.  289) 
suggestive  of  short  non- motile  cilia.  It  is  uncertain  whether  this  is  nor- 
mal or  due  to  disintegration.  Clear  spaces  are  sometimes  seen  in  the 
outer  part  of  the  cells.  The  lumen  is  wide  and  the  cells  are  low  after 
copious  urine  production;  reverse  conditions  occur  when  the  urine  is 
scanty.  It  is  in  the  two  convoluted  portions  of  the  tubules  that  urea  and 
pigments  are  believed  to  be  excreted;  the  fluid  part  of  the  urine  comes 
chiefly  from  the  glomeruli. 

The  descending  limb  both  in  the  radiate  cortex  and  in  the  medulla 
(Figs.  287  and  288)  is  a  thin  walled  conducting  tube  from  9  to  16  /'-  in 
diameter.  (The  proximal  convoluted  tubule  measures  from  40  to  60  //). 
Cell  boundaries  are  absent.     Often  in  sections  the  flat  nucleus  causes 


KIDNEY. 


255 


a  local  thickening  of  the  cell,  but  this  is  perhaps  a  post  mortem  appearance. 
The  descending  hmbs  may  suggest  capillaries  as  seen  in  the  figures. 

The  ascending  limbs,  23-28  i->-  in  diameter,  resemble  the  distal  convo- 
luted tubules  said  to  measure  from  39  to  44  /-<.  The  cells  in  the  distal 
convoluted  tubule  are  taller  than  in  Henle's  loop  and  they  may  have  basal 


^7 


Capsule  of  the 
glomerulus. 


Glomerulus. 


Ascending; limb     \l  y 
of  Henle  s 
loop  ('j 


%^  @- 


0-' 


.'^-^ 


Convoluled  lubule. 


> 


^9rr®y 


Capillary. 


Descending  limb  of  Henle's  loop.  ( 

Small  collecting  tubule. 

Fig.  287.— From   a   Section  through   the   Cortex  of   a   Human  Kidney  (Parallel  with  the 

Surface)  . 
The  pars  radiata  is  seen  in  the  lower  left  corner.     X  200.     (Schaper.) 

striations.     Thus  they  are  much  hke  the  proximal  convoluted  tubules 
except  that  their  markings  are  less  distinct  and  their  size  is  smaller. 

The  collecting  tubules  are  a  distinct  type,  having  a  round,  well  defined 
lumen  and  distinct  cell  walls.  The  round  nuclei  are  arranged  with  striking 
regularity.  The  cells  are  columnar  in  the  papillary  ducts  which  may 
be  0.3  mm.  in  diameter.  Although  some  cells  of  the  collecting  tubules 
appear  darker  than  others,  they  are  thought  to  form  only  conducting  tubes. 


HISTOLOGY. 


From  the  preceding  account  it  is  evident  that  some  parts  of  the  urinary 
tubules  are  easily  recognizable  and  that  others  are  not.  The  capsules, 
descending  limbs  and  the  collecting  tubules  have   distinctive  characters. 


Large  collecting 
tubule. 


•' 


jy 


^-'.     ..,,     '•lilt  1  &; 

Capillary..'    ,.2«  ^>^  „Cj, 


'^"'•'%'         ifcS 


Ascending  limb 
of  Henle's  loop. 


Descending  limb 
of  Henle's  loop. 


Fig.  2SS.— From  a  Transverse  Section  through   the  Medulla  of  a  Human   Kidney.      X  320. 

(Schaper.) 

In  the  medulla,  since  convoluted  tubules  are  absent,  the  ascending  Hmbs 
(including  the  part  of  the  descending  hmb  which  is  of  large  diameter) 
are  hkewise  easily  identified.  In  the  cortex  the  proximal  and  distal  con- 
voluted tubules  wind  about  one  another  and  cannot  be  absolutely  dis- 
tinguished except  by  reconstructions.  In  Fig. 
287,  the  tubules  labelled  ascending  limb  {?), 
found  in  the  radiate  part  of  the  cortex,  have 
also  been  labelled  distal  convoluted  and  end 
segment  of  the  proximal  convoluted;  they  can- 
not be  distinguished  from  these  in  a  single 
section,  but  their  position  in  the  radiate  por- 
tion is  in  favor  of  regarding  them  as  ascend- 
ing limbs. 

The  connective  tissue  about  the  kidney 
forms  a  fatty  capsule,  capsula  adiposa,  which 
surrounds  the  renal  pelvis,  and  its  calyces  ex- 
cept where  they  receive  the  papillae.  A  dense  fibrous  capsule,  tunica  fibrosa, 
is  closely  applied  to  the  outside  of  the  kidney,  from  which  it  may  be  stripped 
oflF.  It  contains  elastic  fibers  which  increase  in  abundance  with  age,  and  also 
smooth  muscle  fibers.  Within  the  kidney  each  tubule  is  surrounded  by 
a  small  amount  of  connective  tissue,  in  part  reticular.     It  is  more  abundant 


>??^TnT,'<'^' 


Fig.  289.  —  Cross  Section  of  .v 
Convoluted  Tubule  fro.m  a 
Rabbit.    (Szymonowicz.) 


BLOOD    VESSELS    OF   THE    KIDNEY.  257 

around  the  vessels,  in  the  papillae,  and  about  the  renal  corpuscles  than 
elsewhere.  The  normal  amount  should  be  carefully  studied  since  an 
increase  in  this  "interstitial  tissue"  is  indicative  of  disease. 

Lobes  and  lobules.  In  embryonic  hfe  the  kidney  is  divided  into  lobes, 
bounded  by  the  renal  columns  and  indicated  by  grooves  upon  the  outer 
surface  (Fig.  290).  The  grooves  become  obliterated  during  the  first  year. 
(In  the  ox  similar  grooves  are  permanent;  in  most  mammals  they  never 
exist,  as  the  kidney  has  but  one  lobe,  papilla  and  pyramid.)  The  lobules 
or  structural  units  of  the  kidney  are  the  areas  centering  around  each  ra- 
diate division  of  the  cortex,  by  which  they  are  drained.  They  are  not 
bounded  by  connective  tissue  septa. 

Blood  vessels.  The  Iddney  has  a  capillary  circulation.  The  renal 
artery,  from  the  aorta,  passes  to  the  hilus  or  notch  on  the  medial 
border  of  the  kidney.  It  divides  into  several  branches 
most  of  which  pass  over  the  ventral  surface  of  the 
pelvis  into  the  fat  around  the  calyces  (Fig.  281).  x\s 
interlobar  arteries  they  pass  to  the  boundary  layer  be- 
tween the  cortex  and  medulla  where  they  are  designated 
arciform  arteries  (Fig.  291).  These  send  interlobular 
arteries  through  the  convolute  part  of  the  cortex  and 
their  terminal  branches  enter  the  fibrous  capsule.  It 
will  be  noted  that  the  kidney  is  exceptional  in  having 

^  ^  o        Fig.  2Q0. — KiDXEY    of 

its  arteries  at  the  periphery  of  its  lobules.     From  the  tAft"r''Hertw/<?r" 

interlobular  arteries  small  stems  pass  to  the  glomer- 
uli, each  of  which  receives  a  single  twig  (Fig.  292).  This  is  resolved 
into  a  knot  of  capillary  loops,  the  endothelium  of  which  seems  to  blend 
with  the  surrounding  s\Ticytium  and  possibly  with  the  inner  layer  of  the 
capsule.  The  glomerulus  often  appears  lobed,  due  to  the  arrangement 
of  its  vascular  loops.  The  capillaries  unite  to  form  a  single  efferent  vessel 
w^hich  divides  into  small  branches  on  leaving  the  capsule.  These  spread 
among  the  convoluted  and  straight  tubules  of  the  cortex  and  some  continue 
into  the  meduUa.  The  latter  is  supphed  by  other  straight  branches 
{arteriolae  rectae)  from  the  interlobular,  efferent  and  arciform  arteries  as 
shown  in  Fig.  291.  The  veins  of  the  meduUa  begin  around  the  papillae 
and  as  venulae  rectae  empty  into  the  arciform  veins.  The  cortical  veins  are 
the  interlobular  vessels  which  are  beside  the  corresponding  arteries.  They 
arise  from  converging  veins  in  the  renal  capsule  which  on  surface  view 
form  a  stellate  figure  {venae  stellatae).  The  interlobular  veins  drain  the 
capillaries  of  the  cortex,  but  have  no  direct  relation  with  the  glomeruh. 
Interlobar  veins  follow  the  arteries,  passing  out  from  the  hilus  of  the 
kidney  over  the  ventral  surface  of  the  renal  pelvis. 
17 


258 


HISTOLOGY. 


Lymphatic  vessels  are  said  to  occur  within  the  cortex  and  to  follow  the 
blood  vessels  out  at  the  hilus.     The  cortical  lymphatics  also  pass  through 


Lobule. 


Lobule. 


Arched  collect 
ing  tubule. 


Tunica  fibrosa. 


—   Stellate  vein. 


• Interlobular 

artery. 

Interlobular 

vein. 


.\rciform 
,-''    artery. 


Arcitorm  vein. 


—  Interlobar  arterv. 


Interlobar  \ein. 


Papillary  duct 


KiG    291.— Diagram  of  the  Course  of  the  Re.val  Blood  Vessels. 


the  tunica  fibrosa  to  connect  with  a  network  in  the  adipose  capsule.     They 
proceed  to  neighboring  lymph  glands. 


XERVES    OF    THE    KIDNEY. 


259 


The  nerves  are  medullated  and  non-medullated.     There  is  a  sym- 
pathetic plexus  at  the  hihis  associated  with  small  ganglia,  and  from  it 


Tunica  fibrosa 
Stellate  vein 
Glomerulus 
Vas  afferens 


— A^  //Elongated 
apillary 
meshes. 


artly  injected 
glomeruli. 
,__  ^fnterlobular  arterj". 
'"Interlobular  vein. 

Fig.  292.— From  a  Section  of  the  Injected  Cortex  of  an  Adult  Hum.an  Kidney.    X  30- 

interlacing  nerves  extend  into  the  kidney  around  the  vessels  (Fig.  293). 
Fine  branches  supply  the  epithelial  cells,  especially  those  of  the  convoluted 


"vr 


T\  Tunica 
'  muscu- 
laris. 

-f'l 


.  (Submu- 
cosa.) 

*  }  Tunica 
r  mucosa 
e) 


t    3 


^0   »    a  ''     ^jr 


Uriniferous  tubules. 

Fig.  293. — From  the  Kidney  of 
A  Mouse.  Golgi  Prepar.a.- 
TioN.    X  180. 


Fig.  294. — Transverse  Section  of  the  Lower  Half  of 
A  HuM.\N  Ureter.    X  i5. 
e.,  Epithelium  ;  t.,  tunica  propria  ;  I,  inner  longitudinal  muscle  bundles 
r,  circular  layer  of  muscle  bundles;  1 1,  outer  longitudinal  muscle 
bundles. 

tubules.     They  form  plexuses  bene  ath  and  above  the  basement  membrane 
and  have  free  intercellular  endingrs. 


26c 


HISTOLOGY. 


Renal  Pelvis  and  Ueetee. 
The  renal  pelvis  and  ureter  both  consist  of  a  mucosa  (and  submucosa), 
muscularis  and  adventitia  (Fig.  294).     The  mucosa  includes  the  epithelium 
and  tunica  propria,  the  latter  blending  with  the  submucosa.     In  sections 


Cylinder  cells  with 
a  cuticular  border. 


LV 


'-^-^,  !y/:. 


'I  Leucocyte. 


-^5__ —   Tunica  propria. 


O- 


dp 


Fig.  295. — \"ertic.-\l  Section  ok  the  Mucous  Membr.ane  of  a  Human  Bladder.     X  560. 


the  epithelium  resembles  that  of  the  moderately  contracted  bladder  (Fig. 
295),  and  its  cells  when  found  detached  in  urine  are  not  distinguishable 
from  bladder  cells.  The  epithelium  is  stratified  but  consists  of  few  layers. 
The  basal  cells  are  rounded,  those  of  the  middle  layer  are  club  shaped 
or  conical  with  rounded  ends,  and  the  outer  cells  are  columnar,  cuboidal, 
or  somewhat  flattened.  Their  lower  surface  may  be 
indented  by  the  rounded  ends  of  several  underlying 
cells,  as  is  particularly  the  case  in  the  contracted  blad- 
der (Fig.  296).  Two  nuclei  are  often  found  in  a 
superficial  cell  and  in  some  animals  they  are  known  to 
arise  by  amitosis.  Leucocytes  frequently  enter  the 
epithelium.  In  some  animals  mucous  glands  have 
been  found  extending  into  the  tunica  propria,  and 
there  are  gland-like  pockets  in  man.  Some  of  these 
have  no  lumen  and  it  is  said  that  none  are  true  glands. 
Capillary  blood  vessels,  which  arc  abundant  in  the 
mucosa,  are  found  directly  beneath  the  epithehum  and 
present  the  deceptive  appearance  of  becoming  intra- 
epithelial. The  tunica  propria  consists  of  fine  connective  or  reticular  tis- 
sue with  few  elastic  fibers.  It  contains  many  cellular  elements  and  some 
leucocytes  and  passes  without  a  definite  boundary  into  the  loose  connective 
tissue  of  the  submucosa. 


Fig.  290.  —  A  Super- 
ficial Epithelial 
Cell  and  Two 
Club-shaped 
Cells  from  a  Con- 
tracted Bladder. 
(Koelliker.J 


BLADDER.  •  26 1 

The  tunica  muscularis  is  not  compact  since  there  is  considerable  con- 
nective tissue  among  its  smooth  muscle  bundles.  The  latter  form  an  inner 
longitudinal  and  an  outer  circular  layer.  In  the  lower  half  of  the  ureter 
there  is  a  third,  outer  longitudinal  layer.  Around  the  papillae  of  the  kid- 
ney the  circular  fibers  form  a  "sphincter."  The  part  of  the  ureter  which 
passes  obliquely  through  the  wall  of  the  bladder  has  only  longitudinal  fibers 
ending  in  the  tunica  propria  of  the  bladder.  By  contracting  they  open  the 
outlet  of  the  ureter.  The  adventitia  consists  of  loose  fibro-elastic  con- 
nective tissue. 

Lymphatics  and  blood  vessels  are  numerous.  There  are  sympathetic 
nerves  to  the  muscles,  and  free  sensory  endings  in  the  tunica  propria  and 
epithelium. 

Pit.  Tangential  sections  of  pits.  Secretion.  Gland. 

/S:i*'S',.  :■  •i'>"--,<<'v". 


^ 


-.-<S£». 


Tunica  propria.  Smooth  muscles. 

Fig.  297. — Section  through  the  Fundus  of  the  Urinary  Bladder  of  .a.n  Adult  Man.    X48. 

Bladder. 

The  development  of  the  bladder  from  the  proximal  end  of  the  allan- 
tois  has  been  described  on  page  193.  Since  the  allantois  is  a  part  of  the 
entodermal  tract,  the  epithehum  of  the  bladder  is  entodermal  whereas 
that  of  the  ureter  is  mesodermal.  There  is  however  no  demarcation 
between  the  layers  in  the  adult,  since  both  produce  the  same  sort  of  "tran- 
sitional epithehum." 

The  bladder  consists  of  a  mucosa,  submucosa,  muscularis  and  serosa 


262  HISTOLOGY. 

The  epithelium  of  the  mucosa  is  two-layered  in  the  distended  bladder,  the 
outer  cells  having  terminal  bars;  in  the  contracted  condition  it  becomes 
several-layered  and  the  bars  form  a  net  extending  into  the  epithelium. 
Some  of  the  superficial  cells  have  a  cuticular  border;  they  often  contain 
two  nuclei  and  their  darkly  granular  protoplasm  has  been  considered 
suggestive  of  secretory  activity.  Round  or  oval  pockets  extend  into  the 
tunica  propria  (Fig.  297).  Some  have  no  lumen  or  are  detached  from 
the  epithelium,  but  others  are  pits  containing  a  colloid  substance.  The 
pits  are  the  first  stages  of  gland  formation.  In  the  adult,  branched  tubules 
Hned  with  cyhndrical  epithehum  may  sprout  from  the  bottom  of  the  pits, 
thus  forming  true  glands.  Their  occurrence  is  hmited  to  the  fundus  (the 
dorsally  bulging  lower  part  of  the  bladder)  and  to  the  neighborhood  of 
the  urethral  outlet.  In  the  latter  position  they  present  transitions  to  well 
developed  prostatic  glands. 

The  tunica  propria  sometimes  contains  soHtary  nodules.  It  blends 
with  the  submucosa,  as  in  the  ureter,  and  contains  lymphatic  and  blood 
vessels,  the  latter  extending  very  close  to  the  epithelium. 

The  muscularis  consists  of  smooth  muscle  fibers  arranged  in  three 
interwoven  layers,  which  are  seldom  separable  in  sections.  They  are  an 
inner  longitudinal,  middle  circular  and  outer  longitudinal  layer.  The 
circular  fibers  are  strengthened  at  the  beginning  of  the  urethra  to  form 
the  "internal  sphincter"  of  the  bladder,  a  muscle  not  always  distinct. 

The  serosa  is  a  connective  tissue  layer  covered  with  mesothehum. 
In  the  non-peritonaeal  part  of  the  bladder  it  is  replaced  by  an  adventitia 
or  fibrous  layer. 

Non-meduUated  nerves,  with  scattered  groups  of  ganglion  cells,  are 
found  outside  of  and  among  the  muscles.  MeduUated  fibers  terminate 
around  the  ganglion  cells;  others  pass  through  the  ganglia  to  intra-epi- 
thehal  sensory  endings. 

Urethra  (in  the  Female). 
The  male  urethra  will  be  described  with  the  genital  organs;  only  its 
upper  portion  is  homologous  wdth  the  urethra  of  the  female  which  is 
exclusively  the  outlet  of  the  urinary  tract.  The  epithehum  has  been 
variously  described  as  stratified,  with  outer  squamous  cells,  or  as  pseudo- 
stratified,  and  columnar.  It  may  be  of  different  form  in  different  indi- 
viduals. The  lumen  is  irregularly  crescentic  with  longitudinal  folds,  as 
seen  in  Fig.  298.  Branched  tubular  urethral  glands  are  found  only  in 
small  numbers  except  near  the  outlet.  Their  secretion  is  mucoid,  but  is 
not  typical  mucus.  In  the  submucosa  there  are  many  thin  walled  veins 
constituting  the  corpus  spongiosum.     It  is  comparable  with  the  upper 


FEMALE   URETHRA. 


263 


part  of  the  more  highly  developed  corpus  cavernosum  urethrae  of  the  male. 
(Compare  with  Fig.  322,  p.  283).  The  muscularis  consists  of  inner  lon- 
gitudinal and  outer  circular  smooth  muscle  fibers,  among  which  the  veins 
extend.  Connective  tissue  with  many  elastic  fibers  is  abundant  in  the 
mucularis.  The  striated  constrictor  urethrae  is  outside  of  the  smooth 
muscle  laver,  as  sho\\Ti  in  the  figure. 


Cf^^*?^>^"-^V 


Fig.  29S. — Cross  Section  of  the  Female  Urethra.     (Koelliker.) 

d.,  Gland-like  diverticulum;   e.,  epithelium  ;   L.,  lumen  of  the  urethra;   m.,  striated  muscle;  s.,  corpus 
spongiosum,  containing  venous  spaces,  v.,  and  smooth  muscle. 


MALE  GENITAL  ORGANS. 


Development. 
The  Wolffian  body  becomes  an  important  part  of  the  male  genital 
organs  and  its  duct  serves  to  transmit  the  products  of  the  testis  to  the 
urogenital  sinus.  Another  duct,  parallel  with  the  Wolffian  and  close 
beside  it,  develops  later,  and  is  called  the  Miillerian  duct.  It  arises  as 
an  inpocketing  of  the  coelomic  epithelium  near  the  anterior  end  of  the 
Wolffian  body.  The  orifice  into  the  peritonaeal  cavity  becomes  surrounded 
by  irregular  folds  known  as  fimbriae.    As  the  Miillerian  duct  grows  poste- 


264 


HISTOLOGY. 


riorly  by  the  elongation  of  its  blind  end,  it  lies  in  contact  with  the  Wolffian 
duct  as  seen  in  Fig.  299,  but  the  Wolffian  duct  is  said  not  to  contribute 
toward  its  formation.  The  two  Miillerian  ducts  reach  the  bladder  side  by 
side  and  acquire  openings  into  it,  between  those  of  the  Wolffian  ducts. 
Near  the  bladder  the  two  Miillerian  ducts  fuse  with  one  another  so  that 
their  distal  part  is  represented  by  a  single  median  tube  on  either  side  of 
which  is  a  Wolffian  duct  (Fig.  279.  B,  page  249).  In  the  female  the  united 
portion  becomes  the  vagina  and  uterus,  and  the  separate  parts  are  the 
uterine  [Fallopian]  tubes.  In  the  male  the  united  portion  becomes  a  small 
blind  pocket,  the  prostatic  utricle,  opening  into  the  prostatic  urethra. 
Each  fimbriated  extremity  persists  in  the  appendix  testis,  and  the  remain- 


u.s. 


Fig.  299.— From  a  Reconstrtction  of  a  13.6  M-^r. 

Human  Embryo.    (F.  W.  Thyng.) 
bl.,   Bladder;    f.,   fimbriae;    g.   g.,    genital  gland ; 

n.  p.,  genital  papilla;    M.   d..  Miillerian   duct ; 

p.,  renal  pelvis;  r.,  rectum;  u.r.,  ureter;  u.  s., 

urogenital  sinus;  W.  d.,  Wolffian  duct. 


Fig.  300. — Diagram  of  the  Development  of 
THE  Testis,  based  upon  Figures  by  Mac- 
Callum   and    B.  M.  Allen. 

C,  glomerular  capsule;  i.  c,  inner  or  sex  cords; 
M.  d.,  Miillerian  duct;  0.  C,  outer  or  rete 
cords  ;  W.  d.,  W.  t.,  Wolffian  duct  and  tubule. 


ing  portion  of  the  ducts,  except  for  occasional  fragments,  becomes  ob- 
literated. Thus  only  the  two  extremities  of  the  ^Miillerian  ducts  are 
ordinarily  permanent  in  the  male  (Fig,  301). 

The  genital  glands  in  either  sex  begin  as  a  thickening  on  the  ventro- 
medial border  of  each  Wolffian  body  (Fig.  299).  A  section  of  this  genital 
ridge  is  shown  in  Fig.  276,  C,  page  245.  The  ridge  is  a  dense  mass  of 
mesoderm  covered  by  the  peritonaeal  mesothelium  w^hich  here  consists 
of  columnar  cells.  In  forming  the  testis,  cords  of  cells  which  later  be- 
come tubules,  appear  in  the  dense  mesenchyma  (Fig.  300).  These  are 
considered  to  be  invaginations  of  the  peritonaeal  layer  rather  than  segre- 
gations of  mesenchyma.  The  cords  near  the  surface  of  the  genital  ridge 
become  the  convoluted  tubules  of  the  testis  {tubuli  contorti)  and  their  con- 


DEVELOPMENT   OF   MALE    GENITAL    ORG.\NS.  205 

tinuations  into  the  substance  of  the  organ  are  the  straight  tubules  {tubuU 
recti).  Both  the  convoluted  and  straight  tubules  (Fig.  301)  arise  from  the 
cords  of  ceUs  in  the  outer  part  of  the  genital  ridge.  The  cords  in  the 
interior  of  the  ridge  are  similar  and  have  recently  been  described  as  the 
posterior  extensions  of  the  rudimentary  peripheral  cords  formed  in  the 
anterior  end  of  the  genital  ridge.  These  inner  cords  produce  a  net  of 
anastomosing  tubes,  the  rete  testis,  into  which  the  straight  tubules  empty. 
The  tubes  of  the  rete  acquire  openings  into  the  glomerular  capsules  of 


sciiiinai   visic.'c 


vrctlira 

appendix  cpididymidis 
appendix    testis 


.convoluted  tubule 


straiHit  tubule 


prostatic  gland 


ductus  deferens 

para  didymis 

in  c  tubes     efferens 


rete  testis 

ductulus   aberrans 
ductus  epididyviidis 


Fig.  301. — Diagram  of  the  Male  Sexual  Organs.     (Modified  from  Eberth,  after  Waldeyer.) 
(The  course  of  the  Miillerian  duct  is  indicated  by  dashes.) 

the" .Wolffian  body  (Fig.  300).  The  glomeruh  atrophy  and  disappear. 
The  products  of  the  convoluted  tubules  thus  pass  in  turn  through  the 
straight  tubules  and  rete  testis  into  the  Wolffian  tubules. 

0/  the  Wolffian  tubules  about  fifteen  persist  as  the  ductuli  efferentes. 
Each  of  these  is  a  greatly  convoluted  tube  which  if  straightened  measures 
8  inches  (20  cms.).  When  coiled  it  forms  a  conical  mass  or  lobule  of  the 
epididymis,  with  its  apex  toward  the  rete,  and  its  base  toward  the  Wolffian 
duct  which  it  enters  (Fig.  301).  The  Wolffian  duct  which  passes  along 
the   dorsal   surface   of   the   testis,  is    also   greatly  convoluted  so  that  it 


266  HISTOLOGY. 

measures  about  20  feet  when  straight  (6-7  meters).  Together  with  the 
efiterent  ducts  this  coiled  mass  constitutes  the  epididymis.  Along  the  testis 
the  Wolffian  duct  is  called  the  ductus  epididymidis  and  from  the  testis 
toward  the  urogenital  sinus  it  is  named  the  ductus  deferens.  Near  its 
termination  a  saccular  outgrowth,  hke  a  distended  gland,  develops  from 
each  Wolffian  duct.  It  is  called  the  seminal  vesicle,  and  that  portion  of 
the  Wolffian  duct  between  the  duct  of  the  vesicle  and  the  urethra  is  named 
the  ejaculaiory  duct.  Thus  the  Wolffian  duct  is  arbitrarily  divided  in 
the  adult  into  three  parts,  the  ductus  epidid)Tnidis,  ductus  deferens,  and 
ductus  ejaculatorius;   an  out-pocketing  forms  the  seminal  vesicle. 

It  has  been  noted  that  only  about  fifteen  of  the  Wolffian  tubules 
persist  as  efferent  ducts.  Some  of  the  others  become  detached,  producing 
the  paradidymis;  and  some  which  are  partly  detached  remain  as  blind 
tubes  extending  from  the  rete  or  ductus  epididymidis, — they  are  called 
ductus  aberrantes.  The  one  of  these  labelled  in  Fig.  301  is  quite  constant 
and  may  be  from  5  to  30  cms.  in  length.  The  appendix  epididymidis  in 
the  figure  contains  a  tube  connected  with  the  Wolffian  duct.  The  nature 
of  this  appendix  is  obscure;  it  has  been  thought  a  derivative  of  the  Miil- 
lerian  duct. 

The  urethra.  At  an  early  stage  (Fig.  299)  the  allantois  is  arbitrarily 
divisible  into  a  'temporary  bladder'  which  extends  to  the  genital  ducts,  and 
a  urogenital  sinus  which  receives  both  urinary  and  sexual  outlets  and 
extends  to  the  surface  of  the  body.  A  portion  of  the  urogenital  sinus  is 
ectodermal  having  formed  from  a  depression  in  the  outer  surface ;  its  inner 
part  is  entodermal  and  the  boundar}'  between  these  portions  is  no  longer 
apparent.  At  a  later  stage  the  'temporary  bladder'  forms  the  permanent 
bladder  together  with  a  limited  portion  of  the  urethra.  In  the  female 
it  forms  the  entire  urethra,  but  in  the  male  only  that  portion  of  the  pros- 
tatic urethra  which  extends  to  the  genital  ducts.  The  remainder  of  the 
male  urethra  is  urogenital  sinus.  By  the  anatomists  the  male  urethra 
is  divided  into  the  prostatic,  membranous  and  cavernous  [penile]  portions. 

The  penis  and  scrotum.  In  Fig.  299  the  outer  portion  of  the  uro- 
genital sinus  is  seen  to  be  a  cleft-hke  space  in  an  elevation  known  as  the 
genital  papilla  (or  tubercle).  In  Fig.  302,  A,  the  papiUa  has  lengthened 
to  form  the  penis;  its  enlarged  distal  end  is  the  glans.  On  the  lo.wer  sur- 
face of  the  penis  the  urogenital  sinus  has  an  elongated  opening.  Apart 
from  the  condition  of  arrested  development  called  h}'pospadias,  the  open- 
ing is  bridged  over,  except  at  its  distal  end;  thus  it  forms  the  cavernous 
part  of  the  urethra.  The  embr}'onic  penis  is  covered  with  a  layer  of  skin 
described  as  forming  two  lateral  folds,  the  lesser  genital  folds.  They  meet 
beneath  the  penis  as  the  urogenital  sinus  becomes  closed,  and  a  raphe 


DESCENT    or   THE    TESTES. 


267 


(seam)  remains  to  indicate  their  place  of  fusion.  A  reduplication  of  tlie 
lesser  folds  over  the  glans  forms  the  -pre-puce.  Outside  of  these  folds  there 
are  two  larger  elevations  of  skin,  one  on  either  side  of  the  root  of  the  penis. 
They  extend  toward  the  anus,  between  which  and  the  penis  they  fuse  in 
the  median  line  forming  a  continuation  of  the  raphe  already  mentioned. 
These  larger  genital  folds  thus  produce  the  scrotum. 

Descent  oj  the  testes.  The  peritonaeal  caxitY  sends  a  prolongation, 
the  processus  vaginalis,  over  the  pubic  bone  into  each  half  of  the  scrotum. 
The  testis  and  epidid}Tnis  at  this  stage  he  behind  the  peritonaeimi  of  the 
abdominal  ca\dt}^  TFig.  302,  B;.  A  large  retroperitonaeal  column  of  con- 
nective tissue,  the  gubernaculum  testis,  extends  from  the  posterior  end  of 
each  testis  into  the  depth  of  the  scrotimi.  For  reasons  still  obscure,  such 
as  unequal  growth  or  the  shortening  of  this  cord,  the  testes  pass  dovra. 
in  front  of  the  pubic  bones,  into  the  scrotum  (Tig.  302,  C).     The  Wolfiian 


Fig.  502.— a,  Diagram  of  the  Embryonic  External  Genital  Organs  in  the  Male;  B,  C,  D, 
Diagrams  of  the  Descent  of  the  Testis.    (After  Eberth.) 

a.,  Anus ;  ep.,  epididymis  ;  g.,  glans  penis  ;  g.  f.,  lesser  genital  folds ;  g.  g.  f.,  greater  genital  folds ;  p.  c, 
peritonaeal  cavity :  p.  v.,  processus  vaginalis ;  r.,  raphe ;  t,,  testis  ;  t.  v.,  tunica  vaginalis  (p.  I.,  parie- 
tal ;  V.  I.,  visceral  layerj :  u.  s.,  urogenital  sinus. 

duct  is  bent  over  the  ureter  as  shown  in  Fig.  301.  Except  on  its  dorsal 
border  the  testis  is  closely  invested  by  the  peritonaeum  of  the  processus 
vaginahs.  Later  the  distal  part  of  the  processus  becomes  separated  from 
the  abdominal  cavity  by  the  obUteration  of  its  stalk.  The  part  remain- 
ing about  the  testis  is  the  tunica  vaginalis,  having  a  parietal  and  a  visceral 
layer  as  shoviTi  in  Fig.  302,  D.  The  descent  of  the  testes  is  completed 
shortly  before  birth  and  the  obhteration  of  the  stalk  of  the  processus 
follows. 


Testis.  ■ 
Sustentacular  and  sexual  cells.  Among  the  cells  of  the  cords  which 
develop  in  the  genital  ridges  there  are  some  which  are  larger  than  the  rest, 
and  are  further  characterized  by  abundant  clear  protoplasm  and  large 
round  nuclei.  Two  of  these  sexual  cells  are  shown  in  Fig.  303,  from  a 
testis  at  birth.     At  this  stage  the  lumen  of  the  convoluted  tubules  is  imper- 


268 


HISTOLOGY. 


fectly  developed  or  absent.  The  sexual  cells  multiply  slowly  by  ordi- 
nary mitosis,  until  puberty  when  their  increase  in  number  becomes  rapid. 
In  a  somewhat  smaller  form  with  round  nuclei  containing  abundant 
chromatin,  in  granules  or  encrusted  at  the  nuclear  membrane,  they  are 
called  spermatogonia.  From  them  the  mature  sexual  cells  are  derived. 
The  cells  which  in  Fig.  303  constitute  the  larger  part  of  the  tubule  are 
called  susieniacular  cells  (Sertoh's  cells,  vegetative,  or  folhcular  cells). 
They  form  a  syncytium  and  with  the  increase  in  the  number  of  spermat- 
ogonia their  protoplasm  is  resolved  into  a  network  of  strands.  Their 
nuclei  are  radially  compressed  into  ovoid  shapes  and  lie  in  columns  of 
protoplasm  extending  from  the  periphery  of  the  tubule  toward  its  lumen 
and  moulded  by  the  surrounding  cells.  Each  nucleus  has  a  distinct  nucle- 
olus apart  from  which  its  chromatic  material  is  very  scanty.  Usually 
the  nuclei  are  in  the  lower  half  of  the  branching  protoplasmic  columns, 


<;3^-2i  «r,:2. 


^1^.^ 


Fig.  303. — Cross  Section  of  a  Convoluted  Fig.  304. — Sustentacular  Cells. 

Tubule    of    the    Testis    at    Birth.        a.,  Isolated  (Koelliker) ;  b.,Golgi  preparations.  (B6hm 

and  von  Davidoff.) 


(Eberth.) 


the  polygonal  bases  of  which  are  in  contact  with  one  another  beneath 
the  spermatogonia.  Within  the  protoplasm  fat  droplets  occur,  together 
with  brown  granules;  crystalloid  bodies  in  pairs  may  also  be  found.  The 
appearance  of  the  sustentacular  cells  in  ordinary  sections  is  shown  in  Figs. 
305  and  309,  in  which  it  is  evident  that  they  may  be  recognized  by  their 
characteristic  nuclei. 

There  are  two  views  as  to  the  origin  of  the  sexual  cells.  According 
to  the  first  they  arise  from  the  mesoderm  quite  like  the  cells  of  other  organs; 
the  second  regards  them  as  a  race  of  undifferentiated  cells  set  apart  from 
the  outset  of  development.  In  the  worm,  Ascaris,  it  has  been  observed 
that  the  fertilized  ovum  divides  into  two  cells,  one  of  which  produces  only 
somatic  cells  (those  of  the  various  tissues)  and  the  other  divides  into  a 
somatic  and  a  sexual  cell.  In  the  mitoses  which  follow,  the  sexual  cell  at  first 
continues  to  give  rise  to  a  somatic  and  a  sexual  cell,  but  later  its  products  are 
wholly  sexual.  In  certain  fishes  large  cells  situated  in  the  entoderm  and  meso- 
derm before  the  genital  glands  have  formed,  are  regarded  as  sexual  cells  (germ 


SEXUAL    CELLS. 


269 


cells).  Later  they  are  scattered  about  in  the  mesothelium  of  the  abdominal 
cavity  and  finally  they  migrate  into  the  genital  ridges  to  become  spermatogonia 
in  the  male,  or  the  corresponding  oogonia  in  the  female.  Similar  cells  have 
been  found  in  reptiles  and  in  the  older  mammalian  embryos.  The  early  segre- 
gation of  the  sex  cells  has  been  cited  in  favor  of  the  opinion  that  acquired  characters 
cannot  be  transmitted;  the  cells  have  been  considered  as  quite  independent 
of  the  body  in  which  they  are  lodged,  which  serves  as  their  "trustee."  It  has 
not  been  established,  however,  that  these  cells  in  mammals  are  earlier  or  more 
completely  separated  from  the  rest  of  the  body  than  are  those  of  other  organs. 

Although  in  consideration  of  the  variety  of  cells  to  which  it  may  give  rise 
the  fertilized  ovum  is  classed  as  the  least  differentiated  of  cells,  yet  the  sexual 
cells  which  unite  to  produce  it  are  highly  differentiated  both  in  form  and  function. 


Spermatids. 
Sustentacula!'  cell. 
Spermatogonium. 


Blood  vessel  with 
blood  corpus- 
cles. 

Interstitial  cells. 


4^ 


Spermatids. 


Sustentacular  cells. 


Sustentacular  cell.        Spermatogonia,  beneath 
large  spermatocytes. 

Fig.  305.— Cross  Sections  of  Seminiferous  (Convoluted)  Tubules  of  a  Mouse.    X  360. 

Neither  of  them  normally  has  the  power  for  further  mitosis,  yet  when  combined 
they  produce  a  cell  in  which  this  capacity  is  unsurpassed.  The  rate  of  cell 
division  falls  as  the  embryo  grows,  and  is  restored  only  in  the  sexual  cells  differ- 
entiated for  this  purpose. 

The  development  of  the  mature  sexual  cells  in  the  male,  the  sper- 
matozoa, occurs  in  the  convoluted  tubules  of  the  testis,  beginning  at  puberty 
and  continuing  throughout  hfe.  With  advancing  age  the  rate  diminishes. 
Since  about  60,000  spermatozoa  occur  in  a  cubic  miUimeter  of  seminal 
fluid,  it  has  been  estimated  that  340  billions  are  produced  in  a  hfetime. 


270  HISTOLOGY. 

The  process  of  their  formation  from  spermatogonia  is  kno\Mi  as  spermat- 
ogenesis. In  place  of  the  name  spermatozoon  which  was  apphed  to 
seminal  filaments  when  they  were  considered  parasitic  organisms,  the 
term  spermium  has  been  proposed. 

Spermatogenesis.     The  spermatogonia  which  are  found  at  the  periph- 
ery of  the  convoluted  tubules,  divide  by  ordinary  mitosis  for  a  variable 
number  of  times.     Some  of  the  resulting  cells  move  toward  the  lumen  and 
increase  considerably  in  size.     The  chromatin  in  the  nucleus  of  each 
forms  a  thread  which  is  resolved  into  one  half  the  usual  number  of  chro- 
mosomes.    Before  this  takes  place  the  chromatin  may  be  gathered  at  one 
side  of  the  nucleus,  a  condition    named  synapsis;     ordinarily  it  appears 
as  a  convoluted    thread  or  spireme.      The  reduced  number  of  chromo- 
somes in  man  is  said  to  be  twelve,  in  favor  of  which  the  drawing  copied 
as  Fig.  306  has  recently  been  published.     The  chromosomes  are  seldom 
so  arranged  as  to  allow  a  conclusive  count.     Instead  of  being  of  the  usual 
elongated  form  they  are  block-hke  bodies  which  in 
certain    animals   are    each  clearly  groups  of    four 
granules    or    subdivisions.       They     become    ring- 
shaped  before  dividing  in  halves  (the   ring-shaped 
arrangement  characterizing  the  heterotypic  form  of 
mitosis)  and  each  half  contains   two    of   the    four 
granules.     The  cells  produced  by  this  division  are 
^  somewhat  smaller    and    pass    toward    the    lumen. 

Fig.  306.— Primary  Sperm-  Within  their  nuclci  the  chromatin  returns   to  the 

ATOCVTE.  HVMAN,  SHOW-  .  r  1  Ml,  ,1  ml 

iNG  12  (?)  Chromosomes.  Spireme   lorm,    and  possibly   to    a   network.     The 

(After  Duesberg.)  ,  .        ,  ,  .      .  ^        ^    •  11 

chromatic  thread  again  is  resolved  into  twelve  chro- 
mosomes, each  in  some  animals  consisting  of  two  granules.  In  the  mitosis 
which  follows,  these  divide  into  single  granules  and  each  of  the  cells  pro- 
duced receives  twelve.  They  then  form  a  network  in  a  small  nucleus,  the 
entire  cells  being  reduced  in  size.  These  cells  border  upon  the  lumen. 
The  generations  of  cells  which  have  been  described  are  named  as  follows. 
Those  which  proceed  from  spermatogonia,  and  which  first  present  the 
reduced  number  of  chromosomes  are  called  primary  spermatocytes.  They 
are  large  cells  in  the  outer  part  of  the  tubule,  sometimes  with  vacuolated 
protoplasm  containing  rows  of  granules.  Each  of  them  divides  into 
two  secondary  spermatocytes  (praespermatids)  which  are  similar  cells, 
though  smaller  and  nearer  the  lumen.  They  also  have  the  reduced 
number  of  chromosomes.  Every  secondary  spermatocyte  divides  into 
two  decidedly  smaller  spermatids,  giving  them  the  reduced  number 
of  chromosomes.  The  spermatids  without  further  division  are  trans- 
formed into  spermatozoa.  Thus  each  primary  spermatocyte  produces 
four  spermatozoa. 


SPERMATOGENESIS. 


271 


Stages  in  the  transformation  of  a  spermatid  into  a  spermatozoon  are 

shown  in   the    diagram    Fig.    307. 

The    twelve    (?)    chromosomes    of 

the  spermatid  disappear  in  a  dense 

chromatic  network  w^hich  becomes 

an  apparently  homogeneous  mass. 

This  deeply  staining  nucleus  passes 

to  one   end  of    the   protoplasm   of 

the     spermatid    and    becomes   the 

essential  part    of   the  head  of  the 

spermatozoon.     In  man  it  is  a  flat- 
tened   structure,   oval    on    surface 

view,  and  pyriform  with   its   apex 

forward,  when  seen  on  edge    (Fig. 

308).     The  head  is  at  the  anterior 

end    of    the    spermatozoon  which 

during  its  development  is  directed 

toward    the    basal    layers    of    the 

convoluted    tubule.      The   anterior 

end  of  the  head  is  probably  covered 

by   a    thin    layer    of    protoplasm, 

known  as  the  galea  capitis.     The 

archoplasm      of      the      spermatid 

(known  as  the  idiozome)  is  said   to    leave  the  centrosome  and  to  enter 

the  protoplasm  of  the  galea  capitis  where  it  forms  the  perforatorium.  If 
this  exists  in  man  it  is  in  the  form  of  a  cutting  edge 
following  the  outline  of  the  front  of  the  head;  in 
other  animals  the  perforatorium  may  be  a  slender 
spiral  or  barbed  projection  which  enables  the 
spermatozoon  to  penetrate  the  ovum. 

The  protoplasm  of  the  spermatid  forms  an 
elongated  mass  at  the  posterior  end  of  the  nucleus. 
It  contains  the  centrosome  which  soon  divides  in 
two.  Of  these  the  anterior  forms  a  disc  which 
becomes  adherent  to  the  nuclear  membrane.  The 
posterior  centrosome  also  becomes  a  disc  after  giv- 
ing rise  to  a  motile  axial  filament  which  grows  out 
from  it  like  a  cilium.  The  disc-like  centrosome 
attached    to    the    anterior    end    of    the    filament 

becomes  thin  in  such  a  way  that  its   peripheral  portion   is  detached, 

and  as  a  ring  surrounding  the  filament  it  passes  to  the  posterior  limit  of 


Fig.  307. — Diagrams  of  the  Development  of 
Spermatozoa.     (After  Meves.) 

a.  c,  anterior  centrosome;  a.  f.,  axial  filament; 
C.  p.,  connecting  piece;  ch.  p.,  chief  piece; 
g.  c,  galea  capitis;  n.,  nucleus;  nk.,neck;  p., 
protoplasm  ;  p.  C,  posterior  centrosome. 


i       Z 


Fig.  308. — Spermatozoa:  1, 
2,  3,  Human;  4,  from 
A  Bull. 

a,  Head  ;  b,  connecting  piece, 
and  c,  chief  piece  of  the 
tail.  1,  3,  and  4,  Surface 
views;  2,  side  view.  X 
360. 


272  HISTOLOGY. 

the  protoplasm.  The  protoplasm  between  the  two  parts  of  the  posterior 
centrosome  is  reduced  to  a  thin  layer  in  which  a  spiral  filament  develops, 
winding  about  the  axial  filament.  The  axial  filament,  which  consists  of 
fine  fibrils  in  some  forms  at  least,  distal  to  the  centrosome  ring  is  sur- 
rounded by  a  thin  membrane  which  terminates  or  becomes  very  thin 
near  the  extremity  of  the  filament.  This  membrane  which  in  salaman- 
ders forms  a  conspicuous  undulating  frill,  is  thought  to  be  a  product  of 
the  filament  and  not  an  extension  of  the  protoplasm.  In  man  it  is  incon- 
spicuous. In  fact  most  of  the  detail  which  is  seen  in  ordinary  sections 
containing  spermatozoa  is  shoxMi  in  Fig.  308. 

Mature  spermatozoa  are  divided  into  three  parts,  the  head,  neck,  and 
tail.  The  head  (3-5  n  long  and  2-3  ij-  wide)  includes  the  nucleus,  galea 
capitis  and  perforatorium.  The  neck  consists  of  the  anterior  centro- 
some and  the  substance,  not  traversed  by  the  axial  filament,  between  it 


Heads  of 
spermatozoa. 


© 


Spermatocyi 


vte_  *V       fe      *,    t-'V*:^        «®%%,a,, 


--'.-       ^    ^^       *^   <^   •  Zp.       *'       ®    «*'     ''^  Nuclei  of  sus- 

V    i^'  •  '.^  '*»,  .  *•  »  S.*     ®  ^  \^^^  tentacular  cells. 


Interstitial  con- 
nective tissue. 


Fig.  309. — From  a  Longitudinal  Section  through  a  Convoluted  Tubule 
OF  A  H  U.MAN  Testis.     X  360. 

and  the  posterior  centrosome.  The  neck  in  man  is  not  constricted  as 
in  some  forms,  yet  it  is  a  place  where  the  head  may  become  detached.  The 
tail  includes  three  parts,  the  connecting  piece,  chief  piece  and  end  piece. 
The  connecting  piece  (6  fj.  long  and  scarcely  i  ii  wide)  consists  of  proto- 
plasm, axial  and  spiral  filaments  and  the  two  parts  of  the  posterior  cen- 
trosome. The  chief  piece  ("40-60  //  long)  is  axial  filament  with  a  surround- 
ing membrane,  and  the  end  piece  (10  y.)  is  a  prolongation  of  the  filament. 
In  the  convoluted  tubules  the  heads  of  the  spermatozoa  are  attached  to, 
or  buried  in  the  protoplasm  of  the  sustentacular  cells  which  are  supposed 
to  nourish  them.  Their  tails  project  into  the  lumen.  Later  they  become 
detached  and  float  in  the  albuminous  fluid  secreted  in  small  quantity  by 
the  tubules.  They  pass  through  the  straight  tubules  and  rete  to  the  epi- 
didymis, in  which  they  accumulate  and  where  they  first  become  motile. 
Their  motility  is  greater,  however,  in  the  seminal  fluid  which  is  a  mix- 


SPERMATOZOA. 


273 


ture  of  the  products  of  the  epididymis,  vesicles,  prostate  and  bulbo-ure- 
thral  glands.  Then  by  an  undulating  movement  of  the  tail  the  head  is 
propelled  against  such  a  current  as  is  made  by  a  ciha,  at  a  rate  of  ^  of 
an  inch  in  a  minute.  Water  inhibits  the  motion,  which  is  favored  by  alka- 
line fluids;  it  occurs  also  m  those  faintly  acid.  Spermatozoa  may  retain 
their  activity  three  days  after  death  and  in  the  female  urogenital  tract 
they  may  hve  a  week  or  more.     In  addition  to  normal  spermatozoa, 


.frr^'-  %=z 


^^''  -i^ 


Fig.  310.— Section  of  the  Human  Rete  Testis.    X  96.    (Kolliker.) 
A,  Artery  i  C,  rete  tubules ;   L,  lymphatic  vessels  ;  s,  connective  tissue  partly  surrounded  bv  rete  tubules  : 
S>k,  part  ot  a  convoluted  tubule,  to  the  left  of  which  are  sections,  probably  of  straight  tubules  •  V.  vein, 
(i'rom  Kaileys     Histology.   ) 

giant  forms  and  some  with  two  heads  or  two  tails  occur,  but  these  are 
of  unkno^vn  significance. 

The  convoluted  tubules  of  the  testis  consist  therefore  of  a  complex 
stratified  cihated  epithehum,  the  basal  ceUs  being  spermatogonia  and  the 
superficial  cells,  spermatozoa.  The  columnar  sustentacular  cells  are 
scattered  through  this  epithehum.  Spermatogenesis  occurs  in  "waves" 
along  these  tubules  as  is  seen  when  they  are  cut  lengthwise  (Fig.  309). 


274 


HISTOLOGY. 


The  superficial  cells  show  alternating  areas  of  mature  and  immature 
spermatozoa.  In  cross  sections  all  the  superficial  cells  may  be  of  one 
stage,  which  differs  from  that  of  the  adjoining  tubule  (Fig.  305).  Toward 
the  periphery  of  the  testis  the  convoluted  tubules  (140  //  in  diameter) 
present  many  loops  and  they  may  anastomose  forming  a  network.  BHnd 
endings  are  also  observed,  and  investigators  disagree  as  to  the  nature  of 
the  usual  termination.  As  they  pass  toward  the  epididymis  they  receive 
branches  at  acute  angles  and  their  windings  diminish.  Sexual  cells  dis- 
appear, leaving    only  the   sustentacular    cells   in    the  form  of    a    simple 


Ductus 
deferens. 


Blood  vessels. 


Epididymis. 


Mediastinum, 
containing  the 

rete  testis. 
Straigiit 
tubules. 

Septula. 


Lobules,  con- 
sisting of  con- 
voluted tubules. 


Tunica 
vasculosa. 

Tunica 
albuginea. 


Fig.  3ri.— Cross  Section  of  the  Testis  of  a  Child  \t  Birth.    X  lo. 

columnar  epithelium.  This  flattens  abruptly  to  form  the  lining  of  the 
straight  tubules. 

A  distinction  between  the  rete  and  straight  tubules  seems  superfluous 
histologically  since  both  are  lined  with  a  simple  epitheUum  of  low  cells. 
In  some  places  these  are  very  flat,  suggesting  endothehum;  in  others  they 
are  columnar.  The  characteristic  dilatations  of  the  rete  tubules  are  shown 
in  Fig.  310.  They  contain  spermatozoa  and  immature  sexual  cells  together 
with  pigment  granules  and  broken  down  cells. 

Connective  tissue  of  the  testis.    The  rete  possesses  no  basement  mem- 


CONNECTIVE   TISSUE   OF   THE   TESTIS.  275 

branes,  such  as  surround  the  convoluted  tubules,  but  is  imbedded  in  a 
mass  of  connective  tissue  known  as  the  mediastinum  testis  (Fig.  311). 
From  the  mediastinum,  layers  of  tissue,  the  septula  testis,  extend  radially 
toward  the  periphery  of  the  testis,  dividing  the  convoluted  tubules  into 
pyramidal  lobules  with  apices  toward  the  rete.  The  periphery  of  the 
testis  is  covered  with  a  dense  connective  tissue  layer,  the  tunica  albuginea. 
It  contains  numerous  elastic  fibers  which  increase  with  age.  The  vis- 
ceral layer  of  the  tunica  vaginalis  rests  upon  its  outer  surface.  The  inner 
portion  of  the  albuginea  is  very  vascular,  forming  a  distinct  layer  at  birth 
(the  tunica  vasculosa).  Connective  tissue  extends  from  the  septula  among 
the  convoluted  tubules.  Immediately  surrounding  them  there  is  a  dehcate 
basement  membrane  followed  by  a  layer  of  closely  interwoven  elastic 
fibers,  and  flat  cells.     In  the  looser  connective  tissue  between  the  tubules 


Interstitial  cells. 


,^  K 


Connective  tissue. 


'"'^'Z^^^^S^^-        ^'%:^ 


__  Convoluted 
^T^ — ^^^X       tubules. 


^■J^iiaS>v^>> 


Fig.  312. — From  a  Cross  Section  of  the  Testis  of  a  Man  Twenty-two  Years  Old.    X  50, 

there  are  clumps  of  interstitial  cells,  shown  in  Figs.  312  and  309.  They 
are  said  to  arise  from  mesenchymal  cells  of  the  genital  ridge.  Some- 
times they  retain  protoplasmic  processes,  but  more  often  they  are  rounded 
or  polygonal  structures  in  close  contact  and  without  distinct  cell  bound- 
aries. In  their  abundant  protoplasm  there  are  pigment  and  other  gran- 
ules, fat  droplets,  and  rod  shaped  crystalloids.  (Rod  and  spindle  shaped 
crystalloids  are  also  found  in  the  spermatogonia  at  all  ages,  and  after 
puberty  octahedral  forms  occur.  Rod  shaped  forms  in  the  sustentacular 
cells  have  already  been  mentioned.  The  composition  and  significance 
of  all  these  are  unknown,  but  they  are  not  considered  post  mortem  for- 
mations.) 

The  interstitial  cells,  although  not  intimately  related  with  the  vessels, 
are  thought  to  produce  an  internal  secretion,  and  there  is  evidence  that 
upon  it  the  sexual  instinct  depends.     During  senile  atrophy  of  the  testis 


276 


HISTOLOGY. 


the  interstitial  cells  at  first  increase ;  later  they  are  destroyed.  At  the  same 
time  the  basement  membrane  becomes  thickened  and  hyahne,  fat  drop- 
lets accumulate,  and  the  sexual  cells  disappear,  leaving  the  sustentacular 
cells. 

The  vessels  and  nerves  of  the  testis  enter  the  mediastinum  and  tunica 
albuginea,  having  followed  the  ductus  deferens  in  the  spermatic  cord. 
The  convoluted  tubules  are  surrounded  by  capillary  networks  derived 
from  branches  of  an  artery  to  a  Wolffian  glomerulus,  and  are  drained  by 
capillary  branches  of  the  Wolffian  sinusoids.  The  main  stems  of  these 
vessels  are  called  internal  spermatic.  Lymphatic  vessels  are  numerous  in 
the  tunica  albuginea  and  they  extend  among  the  tubules.  Nerves  from 
the  spermatic  plexus  accompany  the  blood  vessels;  the  presence  of  intra- 
epithehal  endings  has  not  been  estabhshed  with  certainty. 


Epididymis. 
The  efferent  ducts  which  pass  from  the  rete  to  the  duct  of  the  epididy- 
mis are  hned  with  an  epithelium  in  which  groups  of  columnar  cells  alter- 


^  Tangential  section 
^^'^'^^       of  a  ductulus 
^  efferens. 

■■■}. 


^m^m"'^ 


!feV 


■^■^^^^-\  ;i : ;  ^^%ff^^:,:  •  . : 


Blood  vessel.  Epithelium      Circular  muscles 

of  the  ductus  epididymidis. 


Transverse  section  of  a 
ductulus  eflerens. 


Connective 
tissue. 


Fig.  313. — From  a  Section  of  the  Hkad  of  a  Human  Epididymis,  showing  Sections  of  the 
Dlxtls  Epididymidis  in  the  Center  a.nd  of  Ductlli  Efferentes  on  the  Sides.    X  50. 

nate  with  those  which  are  cuboidal  (Figs.  313  and  314).  Thus  the  inner 
surface  of  the  epithelium  has  depressions  suggesting  glands,  but  the  basal 
surface  is  free  from  outpocketings.  The  epithelium  is  generally  simple, 
although  in  the  tall  parts  it  may  appear  2  or  3  layered.     The  cells  con- 


EPIDIDYMIS. 


277 


Columnar  cells. 


.,J^sg=i        «<^. 


^.■a\I    .\ 


Smooth  muscle  fibers.  Connective  tissue. 

Fig.  314. — Transverse  Section  of  a  Ductulus  Efferens 
Testis  of  an  Adult  Man. 
The  rig-ht-hand  end  of  the  illustration  is  schematic.     Xo  cilia 
could  be  seen,  although  those  of  the  epithelium  of  the  epi- 
didymis were  well  preserved.    X  360. 


tain  fatty,  pigment,  and  other  granules,  and  produce  a  secretion  which  may 
appear  in  vesicular  masses  on  the  surface  of  the  cells.  Often  the  tall  cells 
and  occasionally  the  short  ones  are  ciliated.  The  ciHa  vibrate  so  as  to  pro- 
duce a  current  toward  the  ductus  epididymidis.  The  epithehum  rests  on 
a  striated  basement  membrane  which  is  surrounded  by  a  layer  of  circular 
smooth  muscles,  several 
cells  thick.  The  muscle  cubical  ceiis. 
layer  is  thickest  toward  '^p-^  \  ^^ 
the  ductus  epididymidis.  ^_  I  j 
Among  the  muscle  cells 
there  are  elastic  fibers 
which,  like  those  of  the 
ductus  epididymidis  and 
deferens,  first  appear  at 
puberty.  There  are  no 
glands  in  the  efferent 
ducts,  but  the  irregulari- 
ties in  the  epithelium  are 

thought  to  be  due  to  glandular  activity.     Before  puberty  and  in  old  age 
these  irregularities  are  slight. 

The  ductus  epididymidis  consists  of  a  two-rowed  epithelium  with 
rounded  basal  cells  and  tall  outer  columnar  cells.  The  latter  contain 
secretory  granules  and  sometimes  pigment,  and  have  in  the  middle  of 
their  upper  surfaces  long  non-motile  hairs  which  in  sections  are  usually 

matted   in    conical    processes 
—     ^  (Fig-  ?>?>,  b,  p.  31).     The  epi- 

thelium   may    contain    round 
cavities  opening  into  the  lumen 
or  forming  closed  cysts.     The 
dehcate     membrana     propria 
and  a   thick    circular    muscle 
layer  complete  the  wall  of  the 
ductus,    the    convolutions     of 
which  occur  in  a  loose  connec- 
tive tissue.     Toward  the  ductus 
deferens  the  muscle  layer  thickens.      There  are  no  glands  in  the  ductus 
epididymidis  but  its  cells  produce  considerable  secretion  in  which  the  sper- 
matozoa become  active. 

The  blood  vessels  of  the  epididymis,  which  are  few  in  comparison  with 
those  of  the  testis,  lie  in  part  so  close  to  the  efferent  ducts  as  to  cause  the 
tunica  propria  to  bulge  toward  the  epithehum.     The  nerves,  besides  peri- 


/.  > 


Epithelium. 


Circular  layer  of 
muscle  fibers. 


Loose  connective 
tissue. 


Fig.  315.— Transverse  Section  of  a  Human  Ductus 
Epididv.midis.     X  80. 


278  HISTOLOGY. 

vascular  nets,  form  a  thick  plexus  myospermaticiis  provided  with  sym- 
pathetic ganglia.  It  is  found  in  the  muscular  layer  and  occurs  more  highly 
developed  in  the  ductus  deferens  and  seminal  vesicles.  Its  fibers  supply 
chiefly  the  smooth  muscles,  and  to  a  less  extent,  the  mucosa. 

Ductus  Deferens. 
The  ductus  deferens  begins  as  a  convoluted  tube  continuous  with 
the  ductus  epididymidis ;  it  becomes  straight  and  passes  to  its  termina- 
tion in  the  ductus  ejaculatorius.  Shortly  before  reaching  the  prostate 
it  exhibits  a  spindle  shaped  enlargement,  about  i^  in,  long  and  f  in.  wide, 
kno^^^l  as  the  ampulla  (Fig.  317).     The  ductus  deferens  consists  of  a 

tunica    mucosa,    muscu- 

j^,  laris      and       adventitia. 

-■^'^^^■:t^^:^-ir'^  The  epithehum  is  gen- 

'•■^''^'''      -•■■'^'■'-    j^Sf*^       ..•  Epithelium.  gj.^|Jy    -^^    ^^Q    j-Q^g^     tJ^g 

^' ■  '"  ::*;•-■  tall  inner  cells  producing 

','/  ■  Tunica  propria.  ,  ^^ 

>^'  ..  round  masses   of   secre- 

•,        ,       ,     .,  tion.     Toward   the   epi- 

Iniier  longitu-  i^ 

^^^  •■..-••  dinai  muscles.        didymis  it  may  also  have 

^  Circular  Hon  -  motile    cilia.     To- 


/■—. 


muscles. 


^-  '."';■■><  dinal  muscles. 


ward  the  ampulla  it  may 
■^  '-^  be  several  rowed,  resem- 


•«?>- Outer  longitu-      bUng  the   epithclium  of 


■••::'7%^'  the  bladder.     It  rests  on 


-':^, 


■^•c-  .;  -     :  ,•  .   ... .  J  ••  ■■-^-      """•--  ^°H"1fi"^  a  connective  tissue  tunica 

\V\    -''"^yr^^^^^^'  propria    which    is    sur- 

'  -"  rounded   by    the    three 

F.G.316.-CROSS  Sec™  OF  THE  Hlman  Ductus  \^^tXS  of   the  mUSCUlarfs. 

The  inner  and  outer 
layers  are  longitudinal  and  generally  less  developed  than  the  middle  cir- 
cular layer.  The  adventitia  is  a  loose  elastic  connective  tissue,  blending 
with  that  of  the  spermatic  cord  which  contains  numerous  arteries,  veins, 
lymphatics,  nerves,  striated  muscle  fibers  of  the  cremaster  muscle,  and 
the  rudiment  of  the  processus  vaginaUs. 

In  the  ampulla  the  longitudinal  folds  which  are  low  in  the  ductus 
deferens,  become  tall  and  branched  so  that  they  partly  enclose  irregular 
spaces  (diverticula).  Similar  folds  occur  in  the  seminal  vesicles.  It  is 
doubtful  whether  in  either  place  any  of  them  should  be  considered  glands. 
Around  the  ampulla  the  musculature  is  irregularly  arranged;  the  longi- 
tudinal layers  separate  into  strands  which  terminate  toward  the  ejacu- 
latory  ducts. 


SEillXAL   PASSAGES.  279 

Semestal  Vesicles  axd  Ejaculatory  Ducts. 
The  seminal  vesicles  grow  out  from  the  ductus  deferentes  at  the 
prostatic  ends  of  their  ampullae.  Each  consists  of  a  number  of  saccular 
expansions  arranged  along  the  main  outgro^1;h  which  is  irregularly  coiled. 
The  lining  of  the  sacs  is  honeycombed  with  folds  as  shown  in  Figs.  317 
and  318.  The  epithehum  is  generally  simple  or  two-layered,  the  height 
of  the  cells  varying  with  the  distention  of  the  vesicles  by  secretion.  Gran- 
ules occur  in  the  ceUs,  which  produce  a  clear  gelatinous  secretion  in  sago- 


-m- 


,-  -"'    -      ; 

(--d.d 

/  - 

V/C^^.  ' 

>  -d. 

:'  -    V-  r  "  -  _^-,' 

'., 

-    r  ' 

"--' —  am. 

\  '=ir  ep 


% 


— d.e. 


Fig.  317.  —  Seminal  Vesicle  and  Ductus  Def-  Fig.  31S. — Vertical  Section  of  the  Wall  of 

ERENs.    (This  is  natural  size.)    (After  Eberth.)  a  Seminal  Vesicle.     (After  KoUiker.) 

ad.,  Adventitia ;   am.,  ampulla;   d.,  diverticulum;  ep.,  Simple  epithelium;  g.,  gland-like  depression; 

d.  d.,  ductus  deferens;    d.  e.,  ductus  ejacula-  m.,  muscularis;  t.  p.,  tunica  propria. 

torius  ;   m.,  muscularis;  s.  v.,  seminal  vesicle; 

t.  p.,  tunica  propria. 

like  masses.  Spermatozoa  are  generally  found  in  the  human  vesicles,  but 
except  during  sexual  excitement  they  are  absent  from  the  vesicles  of 
rodents;  this  and  other  facts  indicate  that  the  function  of  the  organ  is 
primarily  glandular.  Pigment  granules  in  varying  quantity  occur  in  the 
epitheHal  cells  and  in  the  underl}dng  connective  tissue.  They  may  im- 
part a  brownish  color  to  the  secretion. 

The  ductus  ejaculatorii  on  their  dorso-median  side  are  beset  ^^'ith  a 
series  of  appendages  which  do  not  project  externally,  but  are  wholly  en- 


28o  HISTOLOGY. 

closed  in  the  connective  tissue  wall  of  the  duct.  Some  of  these  append- 
ages show  the  same  structure  as  the  seminal  vesicles  and  therefore  might 
be  described  as  accessory  seminal  vesicles;  others  are  simply  convolu- 
tions of  alveolo- tubular  glands,  which  may  be  compared  with  prostate 
glands.  The  mucous  membrane  of  the  ductus  ejaculatorii  is  hke  that  of 
the  seminal  vesicles,  except  that  its  folds  are  not  so  complicated.  Muscle 
fibers  occur  only  around  the  appendages.  The  wall  of  the  duct  itself  con- 
sists of  an  inner  dense  layer  of  connective  tissue  with  circular  strands,  and 
an  outer  loose  layer  (adventitia). 

Appendices  and  Paradidymis. 

The  appendix  testis  [hydatid  of  Morgagni,  sessile  hydatid]  is  a  small  vas- 
cular nodule  of  connective  tissue  covered  with  peritonaeum  of  the  tunica  vaginaHs, 
except  at  its  stalk  of  attachment.  It  contains  one  or  more  fragments  of  a  small 
canal,  closed  at  both  ends,  occasionally  having  blind  out- 
pocketings.  The  canals  are  lined  with  simple  columnar 
epithelium  sometimes  ciliated.  The  peritonaeal  cells 
over  a  portion  of  its  siurface  are  columnar  and  have 
been  interpreted  as  the  evaginated  end  of  the  Miiller- 
ian  duct. 

The  appendix  epididymidis  [stalked  hydatid]  is  not 
always  present.  Among  105  cases  examined  by  Toldt 
it  was  found  29  times.  It  consists  of  loose  vascular 
connective  tissue  covered  by  the  vaginalis,  and  contains 
a  dilated  canal  lined  with  columnar  epithelium  some- 
times ciliated.  The  canal  has  no  connection  with  the 
tubules  of  the  epididymis.  Its  embryonic  history  is  ob- 
scure. 
^'''a^  T'^^rs^^'^^nluirai  ^y^^^  iowvid  in  the  vicinity  of  the  epididymis  may 

size.   (After  Eberth.)        arisc  from  pockets  of  the  tunica  vaginalis. 
a.  e.,  Appendix   epididy-  'j^j^g  paradidymis   is   found   "frequently   but   not 

midis;    a.  t.,   appendix  .       /-  ^  H  J 

testis;  c.  e.,  caput  epi-  always  m  older  embryos  and  children,  as  an  elongated, 
tiinfca  vaginai'is.'^'  '  ^"  whitish  Structure  on  the  ventral  side  of  the  spermatic 
cord.  It  is  sometimes  just  above  the  head  of  the  epi- 
didymis, sometimes  higher,  but  always  in  front  of  the  venous  plexus.  A  second, 
lower  part  of  the  paradidymis  is  found  in  late  childhood,  but  not  as  a  rule  in 
the  adult.  It  is  a  macroscopic  coiled  canal  with  outpocketings,  found  behind 
the  head  of  the  epididymis  and  in  front  of  the  pampiniform  plexus."  (Eberth.) 
The  upper  portion  represents  the  anterior  part  of  the  WolflSan  body,  which  is 
not  involved  in  the  formation  of  the  testis.  It  contains  pigment  derived  from  the 
degenerated  Wolffian  glomeruli.  Cilia,  which  occur  at  birth,  later  disappear. 
The  lower  section  may  connect  with  the  tubules  of  epididymis  and  contain 
spermatozoa,  or  it  may  be  completely  detached.  Its  tubules  are  made  of  col- 
umnar epithelium,  simple  or  stratified,  sometimes  ciliated,  and  they  show 
elevations  suggesting  those  of  the  efferent  ducts.     Often  they  become  cystic. 

Prostate. 

The  prostate  consists  of  from  30  to   50  branched  alveolo-tubular 
serous  glands,  which  grow  out  from  the  prostatic  urethra,  and  surround 


PROSTATE. 


2«I 


Smooth 
muscle. 


Glands. 


Connective 
tissue. 


/. 


it  together  witli  the  ejaculatory  ducts  and  the  prostatic  utricle.  The 
prostatic  urethra  is  embryologically  the  neck  of  the  bladder,  and  as  the 
glands  grow  out  they  become  surrounded  by  the  smooth  muscle  fibers 
of  the  bladder  or  urethra.  The  smooth  muscle  of  the  adult  prostate  forms 
a  quarter  of  the  bulk  of  the  organ,  and  together  with  an  elastic  connec- 
tive tissue  it  unites  the  numerous  glands  in  a  compact  mass. 

The  glandular  epithelium  is  simple  and  either  cuboidal  or  columnar. 
It  may  appear  stratified  as  it  passes  over  the  folds  in  the" walls  of  the  tubules. 
Near  the  outlet  of  the  larger  ducts  the  epithehum?_is3_hke  that  of  the 
bladder  and  prostatic  urethra.  In  the 
prostatic  alveoh,"Qf  older  persons  espe- 
ciaUy,  round  or  oval  colloid  masses 
from  0.3  to  i.o  mm.  in  diameter  occur; 
as  seen  in  sections  (Fig.  321)  they 
exhibit  concentric  layers.  Their  re- 
actions on  treatment  with  iodine  solutions 
suggest  amyloid.  These  concretions  are 
probably  deposited  around  fragments 
of  cells.  Octahedral  crystals  also  occur 
in  the  prostatic  secretion,  which  is  a 
thin  milky  emulsion,  faintly  acid;  it 
has  a  characteristic  odor  which  is  ab- 
.  sent  from  the  other  constituents  of  the 
seminal  fluid. 

The  smooth  muscle  fibers  are 
found  everywhere  between  the  pros- 
tatic lobules;  toward  the  urethra  they 
thicken  to  form  the  internal  sphincter  of 
the  bladder.  Smooth  muscle  is  also 
abundant  on  the  surface  of  the  prostate 
and  it  borders  upon  the  striated  fibers 
of  the    sphincter   of    the    membranous 

urethra.  The  prostate  is  abundantly  supphed  ^^ith  blood  and  lymph 
vessels.  The  numerous  nen^es  form  ganghonated  plexuses  from  which 
non-medullated  fibers  pass  to  the  smooth  muscles;  others  of  the  nerves 
have  free  endhigs;  still  others,  both  in  the  outer  and  inner  parts  of  the 
gland  in  dogs  and  cats,  end  in  cyhndrical  lamellar  corpuscles. 


is  ! 

4. 


M. 


n 


-.':^? 


-  \ 


^■^l=5<£  J-**WW 


Fig.  320. — From  a  Section  of  the  Pros- 
tate OF  A  Man  Twenty-three  years 
OLD.    X  50. 


Urethra  and  Penis. 
The  form  of  epithehum  formd  in  the  bladder  extends  through  the 
prostatic  to  the  membranous  part  of  the  urethra.     Its  outer  cells  grad- 


282 


HISTOLOGY. 


ually  become  elongated  and  it  changes  to  the  simple  or  few  layered  col- 
umnar epitheUum  of  the  cavernous  portion.  In  the  dilatation  of  the  ure- 
thra near  its  distal  end,  the  fossa  navicularis,  the  epitheHum  becomes 
stratified  with  its  outer  cells  squamous;  the  underlying  papillae  of  the 
tunica  propria  become  prominent,  and  the  whole  is  the  beginning  of  the 
gradual  transition  from  mucous  membrane  to  skin. 

Glands.  Small  groups  of  mucous  cells  are  scattered  along  the  urethra 
and  in  the  cavernous  part,  especially  on  the  upper  wall,  they  form  pockets 
called  urethral  glands  [of  Littre].     Often  these  pockets  are  on  the  sides 


Red  corpuscles  in  a 
blood  vessel. 


Connective  tissue 


Epithelium 


Smooth 
muscle  fibers. 


Fig.  321. — From  a  Section  of  the  Prostate  of  a  Man  Tv*rENTV-THREE  years  old.     X  360. 
The  epithelium  is  cut  obliquely  at  X,  and  has  artificially  separated  from  the  connective  tissue  at  XX. 


of  epithelial  pits  so  that  the  glands  are  branched.  Non-glandular  pits 
also  occur,  known  as  urethral  lacunae,  and  the  "paraurethral  ducts"  near 
the  external  orifice  are  large  lacunae  of  various  sorts. 

The  two  principal  glands  empty  by  irregularly  dilated  ducts,  an  inch 
and  a  half  long,  into  the  beginning  of  the  cavernous  urethra.  The  bodies 
of  these  bulbourethral  glands  are  found  one  on  cither  side  of  the  mem- 
branous urethra,  in  close  relation  with  striated  and  smooth  muscle  fibers. 
The  end  pieces,  which  are  partly  alveolar  and  partly  tubular,  anastomose. 
They  consist  of  mucous  cells,  with  intercellular  secretory  capillaries,  and 
produce  a  clear,  glairy  mucus,  discharged  during  sexual  excitement.     The 


MALE    URETHRA.  283 

ducts,  surrounded  by  thin  rings  of  smooth  muscles,  consist  of  simple  low 
epithelium.  They  may  connect  directly  with  the  end  pieces,  or  a  secre- 
tory duct  may  intervene. 

The  muscularis  of  the  prostatic  part  of  the  urethra  consists  of  an 
inner  longitudinal  and  an  outer  circular  layer  of  smooth  muscles.  Both 
layers  continue  throughout  the  membranous  part;  the  circular  layer  ends 
in  the  beginning  of  the  cavernous  urethra  leaving  only  oblique  and  lon- 
gitudinal bundles  in  its  distal  part. 

Mucous  membrane  of  the  urethra. 
Epithelium.        Tunica  propria.      Urethral  glands.       Submucosa. 


(W 


Tunica 
albuginea. 

/ 

Arteries.        Connective  tissue  Bundle  of  smooth        Cavernous  spaces, 

trabeculae.  muscle. 

Fig.  322. — Transverse  Section  of  the  Pars  Cavernosa  Urethrae  of  Man.     X  28. 

Corpus  cavernosum  urethrae.  In  the  submucosa  of  the  cavernous 
urethra  there  are  many  veins  (Fig.  322)  which  become  larger  and  more 
numerous  in  and  beyond  the  muscularis.  This  vascular  tissue  which 
surrounds  the  urethra  is  limited  by  a  dense  elastic  connective  tissue  layer, 
the  tunica  albuginea,  and  the  structure  which  is  thus  bounded  is  the  corpus 
cavernosum  urethrae.  Toward  the  perineum  it  ends  in  a  round  enlarge- 
ment, the  bulbus  urethrae,  and  distally  it  terminates  in  the  glans  penis.  The 
urethra  enters  the  upper  surface  of  this  corpus  cavernosum  near  the  bulbus. 
Branches  of  the  internal  pudendal  [pudic]  artery,  namely,  the  arteriea 
bulbi  and  the  urethral  arteries,  penetrate  the   albuginea,  and   the   for- 


284 


HISTOLOGY. 


Fig.  323.— Cross  Section 
OF  AN  Artery  of  the 

BULBUS   U  R  E  T  H  R  A  E, 

SHOWING  Thicken- 
ings of  the  Intima 
AT  X.  Elastic  tissue 
stain.     (After  Eberth.) 


mer  pass  the  length  of  the  cavernous  body  and  end  in  the  glans.  These 
arteries  have  particularly  thick  walls  of  circular  muscle  and  in  cross  sections 
the  intima  may  be  seen  to  form  coarse  rounded  projections  into  the  lumen. 
These  contain  longitudinal  muscles  and  circular  subdivisions  of  the  inner 
elastic  membrane  (Fig.  323).  The  arteries  in  the 
corpus  cavemosum  produce  capillaries  found  chiefly 
toward  the  albuginea.  The  capillaries  empty  into 
thin  walled  venous  spaces  which  appear  as  endothe- 
hum-lined  clefts,  in  a  connective  tissue  containing 
many  smooth  muscle  fibers.  The  cavernous  body 
is  permeated  with  these  veins  which,  at  times  of 
sexual  excitement,  become  distended  with  blood,  re- 
ducing the  tissue  between  them  to  thin  trabeculae. 
Some  arteries  connect  directly  with  the  venous  spaces, 
and  such  as  appear  coiled  or  C  shaped  in  a  collapsed 
condition  are  called  arteriae  helic'mae.  The  venae 
cavernosae  have  such  very  thick  walls  that  they 
resemble  arteries.  They  contain  an  abundance  of  inner  longitudinal 
muscle  fibers  and  since  these  are  not  evenly  distributed  but  occur  in  col- 
umns, the  lumen  of  the  veins  is  usually  crescentic  or  stellate  in  cross  sec- 
tion. Emissary  veins  pass  out  through  the  albuginea  and  empty  into  the 
median  dorsal  vein  of  the  penis. 

The  corpora  cavernosa  penis  are  a 
pair  of  structures  similar  to  the 
cavernous  body  of  the  urethra,  and  are 
found  side  by  side  above  it  (Fig.  324). 
The  septum  between  them  is  perforated 
distally  so  that  they  communicate  with 
one  another.  Each  is  surrounded  by 
a  very  dense  albuginea,  i  mm.  thick, 
divisible  into  an  outer  longitudinal 
and  an  inner  circular  layer  of  fibrous 
tissue.  The  septum  is  formed  by 
the  union  of  these  tunicae.  The 
cavernous  or  erectile  tissue  of  which 
the  corpora  are  composed,  is  essen- 
tially like  that  around  the  urethra. 

All  three  cavernous  bodies  are  surrounded  by  subcutaneous  tissue 
and  fascia,  containing  blood  vessels,  lymphatics  and  nerves,  especially  along 
the  upper  surface  of  the  penis.  The  lymphatic  vessels  form  a  superficial 
and  a  deep  set,  the  latter   receiving    branches  from  the  urethra.     The 


Fig.  324. — Cross  Section  of  a  Penis. 

Skin ;  b.,  subcutaneous  tissue ;  c,  sub- 
fascial tissue;  d.,  dorsal  vein;  e.,  corpora 
cavernosa  penis;  f.,  urethra;  g.,  corpus 
cavemosum  urethrae.     (Bailey.) 


DEVELOPMENT  OF  THE  FEMALE  GENITAL  ORGANS. 


28s 


principal  sensory  nerves  are  the  medullated  dorsal  nerves  of  the  penis. 
They  terminate  in  many  tactile  corpuscles  in  papillae  of  the  skin,  in  bulb- 
ous and  genital  corpuscles  in  the  deeper  connective  tissue,  and  in  lamel- 
lar corpuscles  found  near  or  in  the  cavernous  bodies.  Free  endings  also 
occur.  The  sympathetic  nerves  are  from  a  continuation  of  the  pros- 
tatic plexus.  They  constitute  the  cavernous  plexus,  which  includes  the 
major  cavernous  nerves  accompanying  the  dorsal  nerves  of  the  penis  and 
the  minor  cavernous  nerves  which  enter  the  roots  of  the  corpora  cavernosa 
penis.  The  sympathetic  nerves  supply  the  numerous  smooth  muscles  of 
the  trabeculae  and  cavernous  blood  vessels.  They  are  said  to  be  joined 
by  fibers  from  the  lower  spinal  nerves,  the  nervi  erigentes. 

FEMALE  GENITAL  ORGANS. 

Development. 

During  early  embryonic  development  sex  is  indistinguishable  and 
perhaps  undetermined.  Since  it  is  well  known  that  the  sex  of  mature 
insects  may  be  largely  controlled  by  the  amount  of  nutriment  which  the 
larva  receives,  it  has  been  thought  that  the  sex  of  mammals  may  become 
estabhshed  in  the  course  of  their  embryonic  de- 
velopment. All  attempts  to  find  the  controlling 
factors  have  failed  and  it  is  possible  that  the  sex 
is  determined  when  the  egg  becomes  fertihzed. 

In  both  the  male  and  female  there  are  simi- 
lar primitive  sexual  cells,  genital  ridges,  Wolffian 
and  Miillerian  ducts,  elongated  urogenital  sinuses 
and  prominent  genital  papillae.  The  structures 
shown  in  Fig.  299,  page  264,  may  belong  with 
either  sex.  The  two  Miillerian  ducts  reach  the 
aUantois  side  by  side,  between  the  Wohfian 
ducts.  They  fuse  with  one  another,  beginning 
at  a  short  distance  from  their  outlets  and  ex- 
tending toward  the  allantois  (Fig.  325,  a  detail 
from  Fig.  279,  B,  page  249).  The  fused  portion 
becomes  divisible  into  the  vagina  below  and  the 
uterus  above;  a  thick  layer  of  smooth  muscle  in 

the  mesenchyma  surrounding  the  Miillerian  ducts  characterizes  the  uterus 
(Fig.  326).  A  fold  of  membrane,  the  hymen,  which  is  found  in  the  adult 
at  the  orifice  of  the  vagina,  may  mark  the  termination  of  the  MiiUerian 
ducts.  Some  authorities,  however,  consider  that  more  or  less  of  the  vagina 
is  an  outpocketing  of  the  urogenital  sinus  and  that  the  hymen  has  nothing 


W.d. 


M.d. 


Wd. 


bl.-f 


Fig.  325.  —  Reconstruction 
showing  the  fusion  of 
the  mijllerian  ducts. 
(After  Keibel.) 

bl.,  Bladder;  M.d.,  Miillerian 
duct;  u.,  ureter;  ur.,  ure- 
thra; U.S., urogenital  sinus; 
W.d.,  Wolffian  duct. 


HISTOLOGY. 


uUnne  tub 


cpojpnorcm 


paroof'horou 


to  do  with  the  Miillerian  ducts.  This  opinion  seems  to  rest  on  the  incon- 
clusive evidence  that  persistent  Wolffian  ducts  in  the  adult  may  open  into 
the  vagina  at  some  distance  above  the  hymen. 

The  portions  of  the  Mullerian  ducts  which  do  not  fuse  remain  as  the 
uterine  tubes  (Fallopian  tubes).  Each  opens  freely  through  its  fimbriated 
extremity  into  the  abdominal  cavity.  Cystic  appendages  of  the  fimbriae 
have  been  described,  and  rarely  there  are  accessory  openings  into  the 
peritonaeal  cavity.  The  uterine  tubes,  instead  of  being  vertical  as  in  the 
embryo,  tend  to  become  horizontal.  The  change  in  position  is  associ- 
ated with  the  partial  de- 
scent of  the  ovaries.  The 
ovarian  ligament  and  the 
round  ligament  of  the  uterus 
represent  the  lower  portion 
of  the  genital  ridge  and  the 
gubemaculum  testis  of  the 
male.  The  round  hgament 
is  a  cord  of  connective  tis- 
sue, containing  smooth  and 
striated  muscle  fibers. 

The  Wolffian  tubules 
in  the  female  remain  as 
from  8  to  20  transverse 
ducts,  corresponding  with 
the  ductuli  efferentes. 
They  follow  a  tortuous 
course  from  the  longitudi- 
nal duct  (a  part  of  the  Wolf- 
fian) to  I  the  ovary,  near 
which  they  terminate,  some- 


cliioris 


Fig.  326.- 


'  vestibule 
-Diagram  of  the  F"emale  Genital  Organs. 


times  in  small  cystic  enlarge- 
ments. The  longitudinal  duct,  which  corresponds  with  the  ductus  epididy- 
midis,  ends  bhndly  at  both  ends.  In  from  20  to  60  %  of  cases  it  terminates 
distally  in  a  little  cyst,  the  appendix  vesiculosa,  which  is  lodged  in  a  nod- 
ule of  tissue  attached  to  the  broad  ligament  by  a  slender  pedicle.  Some- 
times there  are  two  or  three  such  appendices.  The  structure  consisting 
of  the  transverse  and  longitudinal  ducts,  which  corresponds  with  the  epi- 
didymis, is  called  the  epoophoron  [no  longer  parovarium  or  organ  of  Ro- 
senmuller].  It  is  a  functionless  renmant  of  the  Wolffian  body  lodged  in 
the  mesentery  of  the  uterine  tube,  where  it  may  easily  be  found.  The 
paroophoron,  a  small  vestige  of  Wolffian  tubules,  occurs  nearer  the  uterus 


DEVELOPilEXT  OF  THE  FE:y:.AXE  GENITAL  ORGAXS, 


€^^ 

^^^v.^ 


% 


3.  327. — Part  of  the   Ovary  at  Birth. 

(Alter  Waldeyer.) 
Epithelium  ;    b,  epithelial  cord  ;    c,   sexual 

cell ;  d,  detached  cord  ;  e,  group  of  follicles ; 

f,  a  single  primary-  follicle  ;  g,  blood  vessel. 

(From  McMurrich.) 


than  the  epoophoron ;  it  has  been  found  in  various  mammals  and  detected 
in  the  human  adult.     Except  for  the  longitudinal  duct,  the  Wolffian  duct 
is  ordinarily  obHterated  in  the  female.     Fragments  may  persist  in  the 
musculature  of  the  uterus  and  these 
"canals  of  Gaertner"  sometimes  open 
into  the  vagina. 

The  ovary,  hke  the  testis,  develops 
from  the  middle  part  of  the  genital 
ridge.  The  upper  end  of  the  ridge  is 
said  to  be  reduced  to  the  band  of 
tissue  {-fimbria  ovarica)  connecting 
the  ovary  with  the  uterine  tube  (Fig. 
326);  except  for  its  ovarian  attach- 
ment this  fimbria  resembles  the 
others.  The  ovary  is  covered  by  a 
layer  of  columnar  peritonaeal  cells 
containing  scattered  large  sexual  cells. 
From  this  layer,  cords  including  cells 
of  both  sorts,  extend  into  the  deeper 
tissue  of  the  genital  ridge  (Fig.  327);  toward  the  epoophoron  their  ar- 
rangement has  been  found  to  suggest  a  rete.  Instead  of  forming  tubules 
which  empty  into  the  Wolffian  body  as  in  the  male,  the  sexual  cords  of 
the  female  produce  detached  islands  of  cells.  The  islands  become 
subdivided  into  groups  usually  containing  a  single 
sexual  cell,  and  knovi-n  as  primary  follicles.  Their 
later  history  will  be  considered  with  the  adult 
ovar}'.  The  rete  cords  become  vestigial  or  dis- 
appear. 

The  urogenital  sinus  which  receives  the  urethra 

and  vagina  becomes    a  shallow   space    called  the 

vestibule   (Fig.   326).     The    genital   papilla,  tipped 

by   its    glans,    becomes   relatively   shorter    as    the 

female    embr}^o    develops.     It   forms    the    clitoris, 

analogous  with  the  penis,  and  is  covered  by   the 

lesser  genital  folds,  the  labia  minora.      (Compare 

Fig.    328    with   Fig.    302,  page   267.)      The   labia 

form  a  prepuce  for  the  chtoris   but   do    not  unite 

beneath  it  making  a  raphe;    they  remain  separate, 

as  parts  of  the  lateral  boundaries  of  the  vestibule.      The  larger  genital 

folds,  labia,  majora,  hkewise  remain  separate.     They   receive   the   ends 

of  the  round  hgaments  of  the  uterus  which  correspond  with  the  guber- 


FiG.  328. — Diagram  of  the 
External  Genital 
Organs  of  a  Female 
Embryo. 

a..  Anus;  g.,  glans  clitori- 
dis ;  g.  f.,  lesser  genital 
folds  (labia  minora); 
g.  g.f.,  greater  genital 
folds  (labia  majora); 
U.S.,  uro-genital  sinus 
(vestibule). 


288  HISTOLOGY. 

nacula  testis,  and  sometimes  the  peritonaeal  cavity  is  prolonged  into 
them  forming  a  processus  vaginalis.  In  late  stages  of  development 
they  become  large  enough  to  conceal  the  clitoris  and  labia  minora 
which  previously  projected  between  them. 

Ovary. 
The  ovary  is  an  oval  body  about  an  inch  and  a  half  long,  covered 
by  a  modified  portion  of  the  peritonaeum.  Along  its  hilus  it  is  attached 
to  a  mesentery,  the  mesovarium,  which  is  a  subdivision  of  the  broad  liga- 
ment of  the  uterus.  The  epithelium  of  the  mesentery  is  continuous  with 
that  of  the  ovary,  and  the  mesenteric  connective  tissue  joins  the  mass 
which  forms  the  central  part  of  the  ovary.     This  tissue,  rich  in  elastic 


.^:= 


Fin.  329.— Cross  Section  of  the  Ovary  of  a  Child  Eight  Years  Old.     X  10. 


ierniinal  epithelium;  2,  tunica  albuginea ;  3,  peripheral  zone  with  primary  follicles ;  4,  vesicular  fol- 
licle I  5,  stroma  ovarii;  6,  mediastinum;  7,  8,  peripheral  sections  of  vesicular  follicles;  9,  hilus,  con- 
taininc  larce  vpin<; 


I,  G 
1 
taining  large  veins 


fibers  and  tortuous  blood  vessels  accompanied  by  strands  of  smooth 
muscle  fibers,  is  sometimes  called  the  medulla  of  the  ovary  but  may  per- 
haps be  better  named  the  mediastinum.  The  peripheral  part,  except  at 
the  hilus,  consists  of  the  connective  tissue  stroma  ovarii  together  with  the 
primary  and  large  vesicular  follicles  which  it  surrounds.  Just  beneath 
the  ovarial  epithehum  it  forms  a  dense  layer  consisting  of  two  or  more 
strata,  the  tunica  albuginea. 

The  formation  of  follicles.  The  germinal  or  peritonaeal  epithelium 
of  the  ovary  consists  of  a  single  layer  of  small  cells  which  may  become  low 
columnar  or  flat.  Even  after  birth  sexual  or  "egg  cells"  may  be  found 
in  it  (Fig.  330).  The  egg  cells  divide  by  ordinary  mitosis  in  the  epithe- 
lium and  in  the  detached  islands  of  peritonaeal  cells  in  the  stroma.  At 
sexual  maturity  nearly  all  of  these  islands  have  been  separated  into  pri- 


FOLLICLES    OF   THE    OVARY. 


289 


Egg:  cell. 


mary  follicles,  each  being  a  single  egg  cell  surrounded  by  a  simple  layer 
of  flat  cells  derived  from  the  peritonaeum.  Sometimes  a  follicle  contains 
two  or  more  egg  cells,  all  but  one  of  which  may  atrophy;  or  the  egg  cell 
may  have  two  nuclei  the  significance  of  which  is  obscure.  The  number 
of  folhcles  in  an  ovary  has  been  estimated  to  be  from  8,000  to  16,000.  Some 
consider  that  no  new  ones 
are  formed  after  birth,  but 
others  beheve  that  they 
may  be  produced  in  the 
adult.  At  all  events  only 
about  200  of  them  become 
mature;  the  others  degene- 
rate at  various  stages  of  de- 
velopment. 

With  further  growth  the  follicular  cells  become  columnar  and  then 
stratified  (Fig.  331);  the  egg  ceUs  enlarge  as  their  protoplasm  becomes 
charged  with  nutritive  material  (yolk  granules  or  deutoplasm).  The 
connective  tissue  around  the  foUicle  is  compressed  to  form  a  distinct  layer, 
the  theca  folliculi.  Later  the  theca  is  divisible  into  a  dense  fibrous  tunica 
externa,  and  a  vascular  tunica  interna  containing  many  cells  with  abun- 


li 


Egg  cells  in 
an  island. 


Egg  cell. 

Nucleolus.  -.^ 
Nucleus.  4— 
Protoplasm. 


Follicular  cells. 


30. — From  a  Section  of  the  Ovary  of  a  Child 
Four  Weeks  Old.    X  240. 


Germinal         Tunica       Primary 
epithelium,    albuginea.     follicle. 


Mitoses. 


Theca 
folliculi. 


A  degenerating 
follicle. 


\c  -a;-'^  b: 


^isastmvi*^^^ 


f^.^^r^ 


^"^^'.^ 


dm^^mmm 


Follicular  cells. 


Nucleus. 


Nucleolus.    Protoplasm.    Zona  pellucida. 


Fig.  331.— From  a  Section  of  a  Rabbit's  Ovary.    X  240. 

dant  protoplasm  (Fig.   332).      A  dehcate  membrana  propria  is  found 

between  it  and  the  folhcular  cells.     After  the  folhcles  have  attained  a 

certain  size  a  crescentic  cleft  appears  among  their  stratified  ceUs.     By 

distention  of  the  cleft  and  enlargement  of  the  foUicle  the  condition  shown 

in  Fig.  332  is  produced.     These  vesicular  follicles  [Graafian  follicles]  vary 

in  diameter  from  0.5  to  12.0  mm.     Besides  the  theca,  the  foUicle  includes 
19 


290 


HISTOLOGY. 


a  stratum  grannlosiim  or  peripheral  layer  of  follicular  cells,  and  the  cumu- 
lus oophorus  or  heap  of  such  cells  containing  the  immature  ovum.  The 
cumulus  is  connected  with  one  side  of  the  folhcle  although  in  certain 


rt—    f  Tunica  externa 
^  2.   I  Tunica  interna. 


£' 


Stratum  granulosum. 


Cumulus  oophorus. 


Egg  cell  with  zona 
pellucida,  nucleus 
and  nucleolus. 


Fig.  332.— Section  of  a  Large  Vesicular  Follicle  of  a  Child  Eight  Years  Old.    X  90. 
The  clear  space  within  the  follicle  contains  the  liquor  folliculi. 

sections  (such  as  a  horizontal  section  near  the  top  of  the  cumulus  in  Fig. 
332)  it  would  appear  completely  detached.  The  columnar  cells  of  the 
cumulus  adjacent  to  the  ovum  are  radially  arranged,  forming  the  corona 

radiata.  The  cavity  of  the  foUicle,  at  first  cres- 
centic,  becomes  so  distended  with  fluid  as  to 
be  nearly  spherical.  The  fluid,  or  liquor  follic- 
uli, is  an  aqueous  transudate  from  the  blood 
vessels.  Certain  appearances  (Call  -  Exner 
bodies)  in  the  stratum  granulosum  have  been 
ascribed  to  cells  undergoing  Hquefaction,  and 
also  to  spaces  containing  a  dense  hquor. 
The  structure  of  the  egg  cell  within  the  cumu- 
lus will  be  considered  under  oogenesis. 

Ovulation  and  the  corpus  luteum.     Around 
the  mature  vesicular  folhcle  the  tunica  interna 
becomes   very   thick   and   cellular,   forming  elevations   toward  the   stra- 
tum   granulosum.      At   this    stage   the  folhcle   is    large,     being    about 
12  mm.  in  diameter,  and    one  surface  of  it  is  so  close  to  the  ovarial 


Fig.  333.  —  OvARV,  Cut  Across, 
Slightly  Reduckd. 

a.,  Aperture  through  which  the 
ovum  escaped;  c.  a. .corpus  al- 
bicans; cl.,  blood  clot  in  a  cor- 
pus luteum  of  ovulation ;  th., 
theca  folliculi ;  v.  f.,  vesicular 
follicle.     (After  Rief^el.) 


OVULATION. 


291 


epithelium  as  to  cause  it  to  bulge  macroscopically  and  then  to  rupture. 
Through  the  opening  thus  made  the  hquor  folhcuH  and  the  egg  cell,  sur- 
rounded by  more  or  less  of  its  corona,  are  expelled  into  the  peritonaeal 
cavity.  The  discharge  of  the  ovum  from  the  follicle  is  known  as  ovula- 
tion. Blood  escapes  from  the  tunica  interna  and  forms  a  clot  within  the 
empty  folhcle  (Fig.  333).  On  all  sides  the  clot  is  surrounded  by  prolifer- 
ating cells  which  contain  a  yellow  fatty  pigment;  thus  they  form  a  corpus 
luteum.  The  lutein  cells  increase  in  size  and  number  and  the  clot  which 
may  show  haematoidin  crystals,  is  gradually  absorbed.  Between  the 
lutein  cells  there  are  strands  of  vascular  connective  tissue  as  shown  in 
Fig.  334.  If  pregnancy  does  not  occur  the  corpus  luteum  attains  its 
maximum  development  in  12  days  and  degenerates  within  a  few  weeks. 


Connective  tissue  septa. 
A 


Fibrous  connective  tissue. 


Vacuoles. 


Lutein  cells. 


Fig.  334. — A,  Portion  of  a  Corpus  Luteum  of  a  Rabbit.    B,  Portion  of  a  Corpus 

Luteum  of  a  Cat.    X  260. 

In  B  the  lutein  cells  have  become  fatty  and  contain  large  and  small  vacuoles. 

Connective  tissue  increases  and  the  lutein  cells  disintegrate;  the  newly 
formed  vessels  are  obhterated  and  the  mass  becomes  a  nodule  of  dense 
"scar  tissue,"  the  corpus  albicans.  If  however,  ovulation  is  followed  by 
pregnancy  the  corpus  luteum  enlarges  even  to  a  diameter  of  from  1.5  to 
3  cms.,  reaching  the  height  of  its  development  in  five  or  six  months.  It 
persists  until  the  end  of  pregnancy.  Thus  the  corpus  luteum  of  preg- 
nancy must  be  distinguished  from  the  corpus  luteum  of  ovulation. 

As  to  the  origin  of  the  granular,  vacuolated  lutein  cells  there  is  a 
difference  of  opinion.  Some  consider  that  they  arise  from  the  stratum 
granulosum,  and  others  from  the  tunica  interna.  They  have  been  com- 
pared with  the  interstitial  cells  of  the  testis,  and  there  is  experimental 
evidence  that  they  produce  an  internal  secretion  without  which  an  embryo 
cannot  develop  within  the  uterus. 


29^ 


HISTOLOGY. 


Many  follicles  degenerate  without  discharging  their  egg  cells.  Cells 
from  the  stratum  granulosum  and  leucocytes  are  said  to  invade  them  and 
after  absorbing  the  egg  protoplasm  they  disintegrate.  The  zona  pellucida, 
a  clear  layer  around  the  egg  cell,  becomes  conspicuously  folded  and  per- 
sists for  some  time  (Fig.  331).  The  basement  membrane  of  the  stratum 
granulosum  has  been  said  also  to  thicken  and  become  convoluted.  These 
degenerating  or  atretic  follicles  are  finally  reduced  to  inconspicuous  scars 
or  they  disappear.  After  the  menopause  the  degeneration  of  the  egg 
cells  becomes  general. 


Fig.  335.  — The  Ovum  as  Discharged  from  a  Vesicular  Follicle  of  an  Excised  Ovary  of 

A  Woman  Thirty  Years  of  Age.    Examined  fresh  in  liquor  foUiculi.    (Nagel.) 

C.  r.,  Corona  radiata  ;  n.,  nucleus  ;  p.,  granular  protoplasm  ;  p.  s.,  perivitelline  space;  y.,yolk; 

z.  p.,  zona  pellucida.     (From  McMurrich's  "  Embr>-ology."j 

Oogenesis.  The  maturation  of  the  ovum  is  comparable  with  that 
of  the  spermatozoon.  Just  as  an  indefinite  number  of  generations  of 
spermatogonia  produced  by  ordinary  mitosis,  terminates  in  primary  sper- 
matocytes, so  the  oogonia  terminate  in  primary  oocytes.  Both  the  primary 
spermatocyte  and  oocyte  give  rise  by  two  reduction  divisions,  in  which 
one  half  the  somatic  number  of  chromosomes  is  involved,  to  four  mature 
sexual  cells.  In  case  of  the  o^um,  however,  only  one  of  the  four  is 
capable  of  fertilization. 

The  sexual  cells  in  the  germinal  epithehum  and  in  the  islands  of  the 


OOGZXESIS. 


29; 


ovary  are  chiefly  oogonia.  The  vesicular  foUicles  contain  oocytes  which 
may  be  recognized  by  their  great  size  (about  200  ;j.  in  diameter).  As 
seen  in  Fig.  335,  the  nucleus  is  large  and  vesicular  [and  is  often  called  the 
germinative  vesicle].  It  contains  a  nucleolus  [germinative  spot]  which 
in  fresh  hquor  foUicuh  exhibits  amoeboid  movements.  The  nucleus  has 
a  distinct  membrane;  usually  it  is  near  the  center  of  the  cell,  but  it  may 
migrate  to  the  periphery.  The  central  part  of  the  protoplasm  contains 
coarse  granules  of  yolk  derived  from  the  fohicular  cehs;  it  is  surrounded 
by  a  finely  granular  zone,  and  this  is  followed  by  a  very  narrow  layer 
free  from  granules.  The  protoplasm  of  oocytes  may  contain  a  "yolk 
nucleus,"  a  structure  formed  by  the  centrosome  and  archoplasm  or  idio- 
zome.  Yolk  nuclei  are  not  found  in  mature  ova.  The  ooc}1:es  probably 
possess  no  distinct  cell  wall.  They  are  surrounded 
by  a  broad,  clear,  radially  striated  band,  the  zona 
peUucida.  The  striations  are  said  to  be  canals 
containing  processes  of  the  follicular  cells.  It  is 
stiU  doubtful  whether  the  zona  is  a  product  of  the 
oocytes  or  of  the  folhcle.  The  egg  cell  may  become 
separated  from  it  by  a  narrow  perhntelline  space  as 
shoT\Ti  m  Fig.  335. 

When  the  primar}'  oocyte  divides  into  the 
secondar}'  oocytes  the  nuclear  material  is  equally 
distributed  between  them.  One  of  them,  how- 
ever, receives  nearly  all  the  protoplasm;  conse- 
quently the  other  is  a  smaU  ceU  and  is  knoTMi  as 
the  first  polar  globule.  In  becoming  a  mature  ovum 
the  secondary  oocyte  di\-ides  for  the  second  and 
last  time,  thus  giving  rise  to  the  ovum  and  second 
polar  globule.  The  first  polar  globule  may  di^dde  in  two.  Thus  the  pri- 
mary oocyte  produces  a  mature  ovum,  and  three  polar  globules  which 
from  their  lack  of  protoplasm  are  generally  fimctionless.  As  they  occur  in 
the  mouse  they  are  shown  beneath  the  zona  peUucida  in  Fig.  336.  It 
is  unknovm  when  the  polar  globules  are  formed  in  man,  whether  in  the 
ovary  before  o\ailation,  or  later.  In  the  mouse  one  forms  in  the  ovary 
and  the  other  in  the  uterine  tube. 

Fertilization.  The  ovum  passes  from  the  peritonaeal  cavit}'  into  the 
fimbriated  end  of  the  uterine  tube,  and  in  the  upper  part  of  the  tube  it  may  be 
fertilized.  The  process  in  man  is  unknown,  but  from  obserA'ations  in  other  ani- 
mals it  is  probable  that  several  spermatozoa  enter  the  zona  peUucida,  and  that 
only  one  passes  into  the  protoplasm  of  the  o\-um.  It  loses  its  taU  piece  as  it 
enters.  The  head  is  resolved  into  tn^elve  (?)  chromosomes  which  become  ar- 
ranged beside  the  twelve  (?)  in  the  nucleus  of  the  o^-um.     The  centrosomes 


Fig.  336.— Ovu.m  of  White 
Mouse.  Scrkounded 
BY  Zona  Pellucida. 

Above  the  ovum  are  two 
polar  globules ;  within 
it  are  two  nuclei,  one 
belong^ing;  to  the  ovum, 
the  other  being  derived 
from  the  head  of  the 
sperniat070on.  X  500. 
l^After  Sobotta,  from 
Minot's  "Embry- 
ology.") 


294  HISTOLOGY. 

of  the  fertilized  o^"^ml  may  be  derived  from  that  of  the  spermatozoon,  or  from 
that  of  the  o\iim.  or  arise  anew;  the  evidence  is  conflicting.  Each  of  the  two 
cells  into  which  the  fertilized  o^"um  divides,  receives  one  half  of  each  of  the 
tn"entv-four  chromosomes,  tv^elve  from  either  parent,  and  in  aU  subsequent  mitoses 
24  (?)  chromosomes  appear.  This  remarkable  distribution  of  chromatin  has 
caused  it  to  be  considered  the  bearer  of  hereditar}-  qualities.  The  spermatozoon, 
however,  contributes  protoplasm  to  the  fertilized  OAimi  and  possibly  the  centre- 
some  also. 

Vessels  and  nenrs.  Branches  of  the  ovarian  and  uterine  arteries 
follow  a  tortuous  course  from  the  hilus  to  the  capillar}-  networks  of  the  tunica 
interna.  They  branch  freely  in  the  stroma.  The  veins  form  a  dense 
plexus  at  the  hilus.  Thin  walled  hTnphatic  vessels  arise  in  the  tunica 
externa  of  the  corpora  lutea  and  larger  follicles,  and  become  more  numerous 
toward  the  hilus.  Their  course  is  independent  of  the  blood  vessels,  peri- 
vascular h-mphatics  being  absent.  There  are  no  hmphatics  in  the  albu- 
ginea.  ^ledullated  and  non-meduUated  nerves  supply  chiefly  the  vessels, 
but  they  form  temunal  nets  in  the  thecae.  It  is  uncertain  whether  any 
extend  among  the  follicular  cells.  Ganglion  cells  have  been  recorded 
near  the  hilus,  but  in  man  the  existence  of  an  ovarian  ganghon  is  denied. 
The  principal  ner\-e  supply  is  the  plexus  0}  the  ovarian  artery. 

Epoophorox. 
The  tubules  of  the  epoophoron  presumably  xsltx  in  structure.  They 
have  been  described  as  cords  of  cells  and  as  tubules  lined  -v^ith  simple 
cuboidal  or  columnar  epithelium,  sometimes  ciliated.  A  layer  of  circular 
muscles  may  surroimd  them  and  internal  longitudinal  fibers  have  been 
fotmd.  The  epoophoron  is  of  interest  as  a  source  of  cysts  of  the  broad 
Hgament.     Peritonaeal  cysts  may  also  occur. 

Uterixi:  Tubes. 
Each  uterine  tube  is  about  5  inches  long  and  extends  from  its  orifice 
in  the  abdominal  ca\ity  to  its  outlet  in  the  uterus.  It  is  diNided  into  the 
fimbriated  funnel  or  injundihulum;  the  ampulla  or  distensible  outer  two 
thirds,  the  lumen  of  which  is  about  a  quarter  of  an  inch  in  diameter;  the 
isthmus  or  narrow  inner  third,  not  sharply  separated  from  the  amptilla; 
and  the  uterine  portion  which  extends  across  the  musculature  of  the  uterus 
to  the  uterine  orifice.  The  tube  includes  a  tunica  mucosa,  (submucosa), 
muscularis,  and  serosa.  The  mucous  membrane  is  thro"SMi  into  folds 
which  are  low  in  the  isthmus  but  are  taU  and  branch  in  the  ampulla,  the 
lumen  of  which  they  seem  to  fill  (Fig.  .337).  The  branches  may  anas- 
tomose; glands  are  absent.  The  ampulla  has  been  compared  with  a 
seminal  vesicle;    in  it  the  ovum  is  probably  fertihzed  normally  and  the 


UTERINE    TUBES. 


295 


Ml 


development  of  large  embryos  within  it  is  not  a  rare  occurrence.  The 
epithelium  is  chiefly  simple  columnar  and  ciliated,  the  stroke  of  the  ciHa 
being  toward  the  uterus.  Small 
areas  of  flat  non-ciliated  cells  may 
occur  near  the  infundibulum  and 
non-ciliated  cells  have  been  found  in 
the  isthmus.  The  tunica  propria  is 
a  vascular  tissue  often  containing 
lymphocytes.  It  extends  into  the 
folds.  In  some  places  the  presence 
of  strands  of  longitudinal  smooth 
muscles  (a  muscularis  mucosae) 
separates  the  mucosa  from  a  sub- 
mucosa.  The  muscularis  consists 
of  a  thick  inner  layer  of  circular 
fibers  and  a  thin  outer  longitudinal 

layer.  The  layers  are  thin  toward  the  infundibulum  where  the  longitudi- 
nal fibers  may  be  absent.  The  loose  inner  tissue  of  the  serosa  is 
sometimes  called  the  adventitia.     Abundant  elastic  fibers  occur  in  it,  and 


Fig.  337.  —  The  Mucosa  of  the  Uterine  Tube 
A,  Near  its  Fimbriated  End;  B,  Near  the 
Uterus.     (After  Orthmann.) 


2*   I 


Serosa. 


\v^. 


\         Longitudinal  muscles. 
->^  \  Blood  vessels. 


Circular  muscles. 


Muco.sa. 
Fig.  338.— Cross  Section,  near  the  Ampulla,  of  a  Uterine  Tube  from  an  Adult  Woman. 


296 


HISTOLOGY. 


/Tube 


except  in   childhood   and  old  age  they  are  numerous  in  the  muscularis 
also.     Blood  vessels  are  highly  developed  between  the  muscle  layers  and 

in  the  mucosa.     The  lymphatics  form  large  ves- 
Fundus  sels  in  the  mesentery  of  the  tube.     Nerves  supply 

the  muscles  and  after  branching  freely  in  the  mu- 
cosa ascend  to  the  epithehum. 

Uterus. 
The  uterus  is  a  muscular,   pyriform  organ, 
flattened  dorso-ventrally.     It  is  about  two  and  a 
half  inches  long,  receiving   the  uterine   tubes   at 
its  upper  end  or  fundus,  and  ending  below  in  the 
vagina.     It  is  divided  into  fundus,  corpus  and 
cervix.     The  triangular  cavity  of  the  corpus  and 
fundus    opens    into    the    canal    0}    the    cervix 
through  the  internal  orifice;  the  canal  communi- 
cates with  the  vagina  through  the   external   orifice  of  the  uterus.     The 
lining  of  the  cervix  presents  a  feather-like  arrangement   of   folds  on  its 
dorsal  and  its  ventral  sur- 


-  Vagina 


Fig.  339. — The  Dorsal  Half 
OF  A  Virgin  Uterus.  % 
natural  size.    (After  Rieffel.) 


.^% 


Mucosa 


r  A 


'^  ]\^: 


r\ 


Muscularis. 


face;  these  are  the  plicae 
palmatae.  The  walls  of  the 
uterus  consist  of  a  tunica 
mucosa,  muscularis  and  se- 
rosa. 

The  thick  "muscularis 
consists  chiefly  of  inter- 
woven circular  and  obhque 
fibers.  A  thinner  outer 
longitudinal  layer  contin- 
uous with  that  of  the  tube, 
is  more  or  less  separated 
from  the  circular  layer  by 
connective  tissue  containing 
many  large  blood  vessels. 
The  outer  layer  borders 
upon  the  serosa  and  is 
sometimes  considered  a  s 
belonging  with  the  subserous 

tissue.  Inside  of  the  circular  layer  an  inner  longitudinal  layer  is  described 
by  Professor  Stohr,  and  the  three  layers  are  said  to  be  quite  distinct  in  the 
cervix.     More  generally  only  two  layers  are  recognized,  an  inner  oblique 


Fig.  340. — From  a  Transverse  Section  of   the   Middle 

OF   THE    UTEULS    OK   A    GiRL    FIFTEEN    YEARS    OlD.       X   10. 

a,  Epithelium  ;  b,  tunica  propria  ;  c,  glands  ;  i,  inner  muscular 
layer;  2,  middle  muscular  layer;  3,  outer  muscular  layer. 


UTERUS. 


297 


Epithelium. 


and  circular,  and  an  outer  longitudinal.  The  uterine  muscles  are  smooth. 
sometimes  branched.  During  pregnancy  they  increase  in  number  and  in 
length  to  three  or  four  times  their  ordinary-  dimensions.  Except  in  the 
peripheral  part  of  its  lower  half  the  uterus  contains  Httle  elastic  tissue. 
There  the  elastic  elements  are  at  right  angles  with  the  course  of  the  muscle 
fibers.  They  increase  during  the  first  haK  of  pregnancy  and  decrease 
in  the  latter  half  (except  in  the  outer  connective  tissue). 

There  is  no  sub  mucosa;  epithehal  pits  or  uterine  glands  extend  to  the 
muscle  layer  and  occasionaRy  enter  it.  They  are  vertical  tubes,  some- 
times branched,  which 
have  a  tortuous  course 
in  their  deeper  part. 
Often  two  or  three 
imite  so  as  to  have  a 
common  outlet.  Their 
distance  from  one 
another,  the  extent  of 
their  flexures  and  their 
relation  to  themuscu- 
laris  are  features  sub- 
ject to  pathological 
changes.  Cystic  dila- 
tations are  common 
especially  in  older 
persons.  The  glands 
produce  no  specific 
secretion.  They  are 
lined  with  simple  col- 
u  m  n  a  r  epithehum 
sometimes  cOiated,  in 
all  respects  hke  that 
of  the  uterine   ca^'it}^ 

Often  ciha  are  absent  from  the  uterine  epithehal  cells,  which  is  said 
not  to  be  due  to  faulty  presen-ation  but  to  the  fact  that  the  cihated  cells 
occur  singly  or  in  groups.  According  to  a  recent  estimate  only  yV  or 
2V  of  the  cells  are  cihated;  and  from  observations  on  certain  animals  it 
is  suggested  that  ciha  are  present  only  in  certain  functional  conditions,  at 
other  times  being  absent. 

In  the  cei^dx,  mucus-producing  cells  occur,  especially  in  the  out- 
pocketings  of  epithehal  pits,  thus  forming  the  branched  ceruical  glands. 
Thev  discharge  a  secretion  which  occludes  the  canal  of  the  cen-ix  diu-ing: 


•-  Gland. 


Mucosa. 


'^■'}i-i^\:§'t-f^\ 


Fig.  34.1.— Mucous  Membr.ajke  of  the  Resting  Uterus  of  .a. 
YouKG  WoM.AJs-.     X  35-     (After  Bohm  and  von  Davidoff.) 


298  HISTOLOGY. 

pregnancy.  Often  they  produce  macroscopic  retention  cysts,  due  to  the 
accumulation  of  secretion  [ovules  of  Naboth,  named  for  the  Leipzig 
anatomist  who  mistook  their  nature  in  1707].  When  empty  of  secretion 
the  cervical  glands  are  said  to  resemble  the  uterine  glands.  Toward  the 
external  orifice  of  the  uterus  the  epithehum  becomes  stratified,  resting  on 
papillae  and  having  its  outer  cells  squamous.  Such  epithehum  is  found 
in  the  vagina,  and  after  the  first  child-birth  it  may  extend  into  the  lower 
half  of  the  cervix. 

The  tunica  propria  of  the  uterus  is  a  very  vascular  reticular  tissue 
with  abundant  nuclei.  It  contains  many  free  lymphocytes  and  its  lym- 
phatic vessels  form  a  wide  meshed  network  with  bhnd  extensions.  They 
empty  into  a  network  of  larger  vessels  in  the  subserous  tissue.  Medul- 
lated  nerv-es  are  said  to  extend  to  the  epithehum  and  many  nonmedul- 
lated  fibers  supply  the  muscularis.  Ganghon  cells  detected  within  the 
uterus  by  the  Golgi  method  are  beheved  to  be  not  more  ganghonic  than 
those  of  the  intestinal  viUi  found  by  the  same  method.  In  the  utero-vagi- 
nal  plexus  which  is  the  source  of  the  sympathetic  nerves  of  the  uterus, 
ganghon  ceUs  have  been  found  in  the  vicinity  of  the  cervix. 

Menstruation. 
Menstruation  is  the  periodic  degeneration  and  removal  of  the  super- 
ficial part  of  the  mucosa  of  the  uterus,  accompanied  by  haemorrhage 
from  the  vessels  of  the  tunica  propria.  For  four  or  five  days  before  the 
discharge  occurs,  the  thickness  of  the  mucosa  increases  due  to  the  conges- 
tion of  its  vessels  and  the  prohferation  of  the  reticular  tissue.  The  glands 
become  ^^ider,  longer,  and  more  tortuous,  opening  between  irregular 
swellings  of  the  superficial  epithelium.  Red  corpuscles  pass  out  between 
the  endothehal  ceUs  of  the  distended  vessels  and  form  subepithehal  masses. 
This  stage  of  tumefaction  is  followed  by  one  of  haemorrhage  and  des- 
quamation lasting  about  four  days.  The  epithehum  of  the  surface  and 
outermost  parts  of  the  glands  becomes  reduced  to  granular  debris,  or  it 
may  be  detached  in  shreds.  The  underlying  vessels  rupture  and  add  to 
the  blood  which  had  escaped  by  diapedesis.  In  the  stage  of  regeneration 
which  requires  about  seven  days,  the  epithehum  spreads  from  the  glands 
over  the  exposed  reticular  tissue,  the  congestion  diminishes,  and  the 
mucosa  returns  to  its  resting  condition.  In  about  twelve  days  the  cycle 
begins  anew.  The  cervix  takes  no  part  in  menstruation  except  that  the 
secretion  of  its  glands  may  increase  during  the  stage  of  congestion. 

'^--  Beginning  at  puberty  (12-15  years)  menstruation  takes  place  normally 
once  in  28  days  for  t,^  years,  more  or  less.  During  pregnancy  it  is  interrupted, 
although  the  time  when  it  should  occur  may  be  indicated  by  slight  uterine  con- 


MENSTRUATION. 


299 


tractions  and  also  by  those  which  cause  the  •  delivery  of  the  child.  Thus  the 
duration  of  pregnancy  is  described  as  ten  menstrual  cycles.  The  significance 
of  menstruation  is  still  obscure.  In  mammals  generally,  a  period  of  congestion 
accompanied  by  uterine  changes  which  are  sometimes  closely  comparable  with 
those  of  menstruation,  precedes  sexual  intercourse  and  ovulation.  Ovulation 
ordinarily  occurs  at  that  time,  independently  of  coitus.  (In  the  rabbit  and 
ferret,  also  in  pigeons,  ovulation  may  fail  to  occur  in  the  absence  of  the  male.) 
In  the  bitch  ovulation  takes  place  when  the  external  bleeding  "is  almost  or  quite 


Disintegrating-  ^'J^^i.^- SX^-X 
epithelium.      ^'■^',^'l^f'l,  ■:'^i.''|t..Vi';:^i,.    .'^%. 

Blood  vessel.   ^^'.xixy^''i^'''''f^Mwf^^^-- 


Excretory  duct. 


f^''^^Ml::-l<''  'uA^vjoli 


•  Superficial  epithelium. 


Disintegrating 
epithelium. 

r:'<-.'-='-7'---\-"  Pit-like.depression. 


\'-  ■ ; i/^yf-E  -M~:-^:^,ir'.'-':^'i^-*"i7:'' —  Excretory  duct. 


^vn\^ 


'<'•'■:■( '''':'': 


;.-a!^-"-:4_.  Gland  tubule. 


Dilated  tubule. 


'^s-: 


Blood  vessel. 


Blood  vessel. 


'■'■  I 


Muscularis. 


Fig.  342.— Mucous  Membrane  of  a  Virgin  Uterus  During  the  First  Day 
OF  Menstruation.    X  30.    (Schaper.j 

over,"  and  this  is  the  time  of  coitus.  Domestication  in  various  animals  causes 
an  increased  frequency  of  the  congestive  cycles,  sometimes  unaccompanied  by 
ovulation.  It  is  generally  accepted  that  human  ovulation  is  independent  of 
coitus  and  to  some  extent  of  menstruation.  The  spermatozoa  of  rabbits  retain 
their  activity  and  are  capable  of  fertilizing  the  ovum  for  about  ten  days,  and 
it  is  perhaps  true  that  if  human  ovulation  takes  place  within  some  such  period 
after  coitus,  fertilization  may  occur.  The  ovum  is  said  to  take  four  days  in  the 
rabbit  and  eight  or  ten  in  the  bitch  to  pass  through  the  tube  to  the  uterus.     The 


300 


HISTOLOGY. 


condition  of  the  mucosa  of  the  human  uterus  when  the  fertihzed  ovum  enters 
it  is  unknown.  The  stage  of  development  of  many  young  human  embryos  sug- 
gests that  their  growth  began  nearer  the  time  of  the  first  menstruation  which 
lapsed  than  the  last  which  occurred.  This  may  be  due  to  the  frequency  of 
human  menstruation,  which  may  still  be  preparatory  to  coitus  as  in  other 
mammals. 

The  Development  of  the  Decidual  Membranes. 
Before  describing  the  mucosa  of  the  uterus  during  pregnancy,  it  is 
necessary  to  consider  the  membranes  of  the  embryo  which  are  in  contact 
with  it.  Fig.  343,  A,  represents  a  blastodermic  vesicle  in  which  the  three 
germ  layers  are  present.  (The  formation  of  such  a  vesicle  by  the  seg- 
mentation of  the  ovum  has  been  figured  on  page  19.)  In  a  thickened 
portion  of  the  outer  layer  of  the  vesicle  a  cleft  occurs,  which  in  B  has 
widenedfand  become  the  amniotic  cavity.     It  is  bounded  below  by  the 


Fig.  343.— Three  Diagrams  of  the  Hypothetical  Development  of  the  Human 

Decidual  Membranes.     (After  Minot.) 

al.,  Allantois;  am.,  amnion  ;  am.  C,  amniotic  cavity;  cho.,  chorion  ;  coe.,coelom;  y.  s.,  yolk  sac.    The 

mesoderm  is  stippled,  the  ectoderm  is  shaded  with  lines  and  the  entoderm  with  dots. 

ectoderm  which  covers  the  body  of  the  embryo,  and  above  by  a  layer 
which  is  soon  divided  into  two  parts  by  an  extension  of  the  body  cavity. 
This  has  occurred  in  C.  The  inner  layer  or  amnion  consists  of  ectoderm 
toward  the  embryo  and  mesoderm  away  from  it.  It  is  a  membrane  con- 
tinuous with  the  skin  of  the  embryo.  The  outer  layer  or  chorion  surrounds 
the  entire  vesicle  and  is  characterized  by  shaggy  villi.  It  consists  of  ecto- 
derm [trophoblast]  on  its  peripheral  surface  and  mesoderm  within. 
A  stalk  of  mesenchymal  tissue  surrounding  the  allantois  extends  from  the 
embryo  to  the  chorion.  It  lodges  the  umbilical  (allantoic)  vessels  through 
whieh  the  blood  of  the  embryo  passes  to  the  chorionic  villi  and  back  to 
the  embryo.  These  villi  enter  into  close  relation  with  uterine  mucosa, 
being  bathed  in  maternal  blood,  and  the  embryo  receives  such  nutriment 
as  is  absorbed  through  their  walls.     Human  embryos  of  the  stage  C  are 


DECIDUAL    MEMBBANES. 


301 


well  known,  but  the  youngest  which  have  been  obtained  are  more  advanced 
than  B ;   therefore  the  stages  A  and  B  are  hypothetical. 

In  further  development  the  amniotic  ca\dty  enlarges  so  that  the  am- 
nion is  in  contact  with  the  inner  surface  of  the  chorion  (Fig.  344).  The 
yolk  sac  and  its  attenuate  stalk  are  brought  close  to  the  allantois.  The 
mesenchymal  tissue  surrounding  the  yolk  stalk  and  allantois  and  covered 
by  a  layer  of  ectoderm,  forms  the  umhilical  cord.  At  first  it  contains  an 
extension  of  the  body  cavity  around  the  yolk  stalk  but  later  this  is  obHter- 
ated  by  adhesions.  The  ectoderm  of  the  cord  is  continuous  distally 
with  that  of  the  amnion  and  proximally  with  the  epidermis  of  the  embryo. 
There  is  an  abrupt  transition  from  the  skin  with  its  capillaries  to  the  non- 
vascular covering  of  the  cord,  which  at  birth  is  about  8  mm.  from  the 
abdominal  wall. 

Only  one  side  of  the  chorionic 
vesicle  becomes  implanted  upon  the 
uterine  mucosa.  The  vilh  on  that  side 
of  the  vesicle  proHferate  and  constitute 
the  chorion  frondosum.  Elsewhere 
the  vilh  become  scattered  and  low, 
finaUy  disappearing;  the  resulting 
smooth  part  of  the  chorion  is  called 
the  chorion  laeve. 

The  appearance  of  a  human  em- 
bryo at  a  stage  intermediate  between 
the  last  tw^o  diagrams  considered,  is 
shown  in  Fig.  345.  The  greater  part 
of  the  villous   chorion   has   been   cut 

away  together  with  half  of  the  thin  smooth  .amnion,  thus   exposing   the 
embryo  with  its  umbihcal  cord  and  yolk  sac. 

Relation  between  the  membranes  and  'the  uterus.  That  portion  of  the 
uterine  mucosa  against  which  the  chorionic  vesicle  rests  and  into  which 
its  villi  prohferate,  is  called  the  decidua  basalis  [serotina].  A  portion  which 
grows  over  the  vesicle  completely  enclosing  it  is  the  decidua  capsularis 
[reflexa].  The  remainder  of  the  mucosa  is  the  decidua  vera  (Fig.  346). 
As  the  embryo  increases  in  size  so  as  to  fill  and  distend  the  uterine  cavity, 
the  decidua  capsularis  becomes  thin,  degenerates,  and  is  resorbed  so  that 
in  the  last  half  of  pregnancy  the  chorion  laeve  rests  directly  upon  the 
decidua  vera  (Fig.  346,  B).  The  chorion  frondosum  together  with  the 
inseparable  part  of  the  decidua  basahs  into  which  its  villi  have  grown, 
form  the  placenta.  Thus  the  placenta  consists  of  a  uterine  and  a  fetal 
portion.     It  is  a  discoid  mass  of  vascular  tissue  which  at  birth  is  about 


Fig.  344. — Diagram  of  the  Formation  of 
THE  Umbilical  Cord,  Lettered  as  in 
Fig.  343- 


302 


HISTOLOGY. 


7  inches  in  diameter,  i  inch  thick,  and  weighs  a  pound.  The  distal  end 
of  the  cord  is  usually  but  not  always  inserted  near  its  center.  From  the 
end  of  the  cord  the  amnion  spreads  over  the  placenta  and  is  lightly  adherent 
to  it;    the  free  surface  of  the  amnion  is  smooth  and  glistening.      The 


cho.  ^  _  _ 


i4  ^v 


"'"  '";*?*- 


Fig.  345. — A  Normal  Human   Embryo  of   io.o  mm.,  Ricmoved  Surc.icallv  with  the  Uterus,  Six 

Weeks  after  the  Last  Menstruation. 

The  embryo  has  been  exposed  by  cutting  away  most  of  the  chorion,  cho.,  and  part  of  the  amnion,  am.; 

U.C,  umbilical  cord  ;  v.,  chorionic  villi ;  y.S.,  yolk  sac. 

chorion  lacve,  beginning  at  the  placental  margin,  continues  clear  around 
the  cavity  of  the  uterus  and,  as  before  mentioned,  the  amnion  adheres 
to  it.  The  amniotic  cavity  is  filled  with  fluid  in  which  the  embryo  is 
immersed.  Shortly  before  birth  the  cervix  dilates  and  the  membranes 
thus  exposed,  rupture.     The  amniotic  fluid  escapes  and  the  child  follows, 


DECIDUAL    iIEMBR.lXES. 


303 


its  umbilical  cord  extending  through,  the  vagina  to  the  placenta.  In  the 
course  of  half  an  hour  the  placenta  and  membranes  are  expelled,  the  sac 
which  they  form  being  inverted  in  the  process.  Thus  the  smooth  or 
amniotic  surface  of  the  placenta  is  exposed.  The  very  thin  membranes 
attached  to  its  margin  consist  of  amnion,  chorion  and  fragments  of  decidua 
vera.  The  denuded  uterine  mucosa  is  gradually  restored  to  its  normal 
condition,  as  after  menstruation.  EpitheHum  spreads  over  its  surface 
from  the  bases  of  the  glands.  In  the  following  account  the  histology  of 
the  decidua  vera  and  adjacent  membranes  will  be  considered  first,  then 
the  placenta  and  finally  the  cord. 


Fig.  346. — The  Uterus  and  Decidual   Membranes   in   Early  Pregnancy,  A,  and  in  Late  Preg- 
nancy, B.    The  Cord  has  been  Cut  and  the  Embryo  Removed  from  B. 

am.,  Amnion;  am.  C,  amniotic  cavity;  c,  cervix;  ch.,  chorion  ;  c.  U.,  cavity  of  the  uterus;  d.  b.,  decidua 
basalis;  d.  c,  decidua  capsularis;  d.  v.,  decidua  vera ;  m.,  amnion  and  chorion  laeve  drawn  as  one 
line ;  pi.,  placenta  ;  u.  C,  umbilical  cord  ;  y.  s.,  yolk  sac. 


DeCEDUA   VeILA.,    AiESTIOX,    AXD    CHORION   LaEV'E. 

On  the  upper  surface  of  the  section  Fig.  347,  is  seen  the  amnion, 
having  its  simple  cuboidal  or  flat  epithehum  toward  the  embryo,  and  its 
mesodermic  connective  tissue  toward  the  chorion.  Adhesions  in  the 
form  of  slender  strands  bind  it  to  the  connective  tissue  of  the  chorion. 
The  chorionic  epithehum  forms  a  layer  over  the  surface  of  the  vera;  it 
presents  shght  irregularities  but  is  without  "\dlh.  The  superficial  uterine 
epithehum  has  degenerated;  it  disappeared  in  an  earher  stage.  The  modi- 
fied mucosa  or  decidua  vera  is  divisible  into  a  superficial  compact  layer 
and  a  deep  cavernous  layer.  After  the  epithehum  of  the  glands  in  the 
compact  layer  degenerated  and  was  resorbed,  the  connective  tissue  came 
together  obhterating  the  gland  cavities.     The  compact  layer  is  therefore 


304 


HISTOLOGY. 


Amnion.        ?"'?32^»?' 


Chorion 


Compact  layer. 


Cavernous  layer.    < 


Muscularis. 


Fig.  347. — Vertical  Section  through  the  Wall  of  a  Uterus  about  Seven  Months  Pregnant, 
WITH  THE  Fetal  Membranes  in  Situ.  Between  amnion  and  chorion  are  threads  of  gelatinous 
connective  tissue.     X  30.     (Schaper.) 


without  glands. 


Fig.  348.— Decidual  Cells  from  the 
Mucous  Membrane  of  a  Human- 
Uterus  about  Seven  Months 
Pregnant.  Below  a  "giant-cell," 
above  to  the  right  a  cell  with  a  kary- 
okinetic  figure.    X  250.    (Schaper.) 


The  cells  of  the  tunica  propria  have  enlarged,  and  become 
decidual  cells  (Fig.  348).  These  cells  which 
occur  only  in  pregnancy  are  flattened,  round, 
oval  or  branched  structures  of  large  size  (.03 
to  0.1  mm.).  Usually  they  contain  a  single 
nucleus  but  often  there  are  two  or  more,  and 
in  giant  forms  there  may  be  30  or  40.  The 
cavernous  layer  of  the  mucosa  contains  slen- 
der clefts  parallel  with  the  muscularis. 
These  are  glands  which  have  been  stretched 
laterally;  some  of  them  retain  areas 
of  normal  epithelium,  but  in  many 
the  epithelium  has  degenerated  and  from 
some  it  has  wholly  disappeared.  The  con- 
nective tissue  is  but  slightly  modified. 
Throughout  the  decidua  but  especially  in 
the  superficial  portion,  the  vessels  are 
greatly  distended. 


PLACENTA. 


305 


Placexta. 
The  chorionic  vilh,  the  interlacing  branches  of  Avhich  form  the  fetal 
portion  of  the  placenta,  are  shaped  as  shovm  in  Fig.  349.  The  finding 
of  such  structures  in  a  uterine  discharge  or  curetting  is  diagnostic  of 
pregnancy.  The  villi  in  the  earliest  stages  are  composed  entirely  of  epi- 
theHum,  but  they  soon  acquire  a  core  of  the  chorionic  mesench}Tiial  tissue 
in  which  are  the  terminal  branches  of  the  umbihcal  vessels.  The  epithe- 
hum  is  very  early  di^dsible  into  two  layers.  The  outer  layer  consists  of 
densely  staining  protoplasm  containing  dark  round  or  flattened  nuclei. 
Since  cell  boundaries  are  lacking,  this  is  called  the  syncytial  layer.  ^Mitoses 
are  seldom  seen  in  it.     Generally  its  nuclei  are  in  a  single  layer  but  they 


Fig.  349. — Isolated  Terminal  Branches  of   Chorionic  Villi  ;  that  on  the   Left  is  from  an 
Embryo  of  Twelve  Weeks;  on  the  Right  at  Full  Term.    (Minot.) 


may  accumulate  in  "knots"  or  "proHferation  islands,"  especially  in  late 
stages.  The  knots  project  from  the  surface  of  the  \'iUi  so  that  in  certain 
planes  of  section  they  appear  completely  detached  and  suggest  multinu- 
cleate giant  cells.  The  S}Ticytial  layer  perhaps  completely  invests  the 
villi  at  first,  but  later  it  is  interrupted  in  many  places. 

The  deeper  layer  of  the  chorionic  epithehum  consists  of  distinct  cells 
with  round  nuclei  and  clear  protoplasm.  Although  this  is  a  single  layer 
at  the  base  of  young  viUi,  it  produces  great  masses  of  cells  at  their  tips. 
These  columns  or  caps  of  cells  in  which  the  vilh  terminate,  fuse  with  one 
another  next  the  decidua,  and  the  uterine  tissue  seems  to  be  dissolved  as 
this  mass  of  epithehum  prohferates.     All   the  superficial  epithehum  of 


3o6  HISTOLOGY. 

the  decidua]^basalis  degenerates  and  disappears,  and  the  distal  parts  of 


Syncytium. 


Cuboidal  cells  of  the  basal 
layer. 


Connective  tissue. 


^  P- ''t^-Jj^^'^ -Blood    vessel    containing 

^^^■^      '®i     ^  nucleated  red  corpus- 


Oblique  section  of  the  epithelium. 
Fig.  350. — Cross  Section  of  a  Human  Chorionic  Villus  of  the  Fourth  Week  of  Pregnancy. 

the  blood  vessels  in  the  tunica  propria  are  destroyed.  The  uterine  blood 
escapes  into  the  intervillous  spaces,  bounded  by  the  syncytium,  or  where 
this  is  deficient,  by  the  basal  cells.     The  maternal  blood  circulates  in  the 


Amnion.  --<t.-™ 


Chorionic  villi. 


*-  'W^"^"^^  (^^ 


Compact 
layer. 


Cavernous 
^    L        layer. 


Muscularis. -j 


'  r^         Ui^i         }  ^    Intervillous  spa 
'—'    Floating  villus. 


.^><^. 


^  Si^ 


i  Attached  villi. 
Vein. 

Spiral  artery, 
(".land. 


f/     \'eiii. 

i-:./^:---€L'ilSilU--' '^     '--  :  '-'5' 

Fig.  351.— Diagram  of  the  Human  Placenta  ai    riii-,  Cios)-:  01    I'ricgnancv.     (Scliapcr). 


intervillous  spaces  as  shown  in  the  diagram  Fig.  351,  and  does  not  clot. 


PLACENTA. 


307 


So  extraordinary  is  this  that  attempts  have  been  made  to  detect  an  endo- 
thehal  covering  for  the  vilh,  but  without  success.  (The  syncytial  layer 
has  been  considered  endothelial  or  otherwise  of  maternal  origin,  but  this 
view  is  not  accepted.) 

The  placenta  at  birth,  being  an  inch  thick,  presents  in  cross  section  a 
vast  number  of  the  branches  of  villi  cut  in  various  planes.  In  the  villi 
of  Fig.  352  it  is  seen  that  the  epithelium  is  in  places  hardly  distinguishable 
from  the  connective  tissue.  This  thin  portion  may  represent  the  basal 
layer  and  the  dark  clumps  of  nuclei  scattered  over  its  surface  may  arise 
from  the  syncytium,  but  the  reverse  relation  of  the  two  types  of  epithe- 
lium to  the  original  layers  is  sometimes  stated.  Within  the  "vdllus  are  the 
blood  vessels  of  the  embrvo;    their  blood  never  mixes  with  the  maternal 


Epithelium 
Epithelial  nucleus 

Capillaries,  •ej 


Syncytial  knot. 


Small  arterj-. r^^ 


Syncytial  knot. 


Epithelium. 


-I-- Small  vein. 


'-"%--'-*|- Capillary. 


Syncytial  knot 


Fig.  352.— Cross-section  through  a  Smaller  (A)  and  a  Larger  (B)  Chorionic  Villus  of  a 
Human  Placenta  at  the  End  of  Pregnancy.    X  250.     (Schaper.) 

blood  which  surrounds  the  villi,  as  is  easily  seen  in  the  early  stages  when 
the  fetal  blood  contains  nucleated  red  corpuscles. 

The  embryonic  surface  of  the  placenta  is  sho^^^l  in  Fig.  353.  Along 
the  chorionic  epithehum  there  are  generally  areas  of  hyahne  material 
which  stain  deeply  with  eosin  and  have  the  appearance  of  fibrin.  In  the 
outer  part  of  the  placenta  also,  the  viUi  may  seem  to  terminate  in  hyaline 
masses  attributable  to  the  degeneration  of  the  inner  epithehum.  The 
hyahne  masses  [canalized  fibrin]  are  a  conspicuous  feature  of  the  mature 
placenta. 

The  decidua  basalis  consists  of  compact  and  cavernous  layers,  tliin- 
ner  but  similar  to  those  of  the  vera  (Fig.  354).  It  sends  septa  into  the 
fetal  part  of  the  placenta  dividing  it  into  coarse  lobes  or  "cotyledons." 


3o8  HISTOLOGY, 

The  maternal  arteries  are  in  the  septa  but  the  veins  are  in  the  spaces 
between  them. 

^<<fS~"?»-- 

Choiioiiic  \ 


-Blood 

vessel. 

-     \< 

■d  corpuscles. 

- 

mnective  tissue. 

Si 

.  iic>tium. 

1 

-     Hyalin. 

■  i 

1 
i 

^  Deep  layer  of  chori- 
onic epithelium. 

Lunnective  tissue. 
~  Leucocytes. 


'i::-^ 


tte 


% 


1 

.  -      Homogeneous  layer. 
!•< —  Amniotic  epithelium. 

I''"-'-  353- — 1'"Rom  a  Cross-section  of  a  Mature  Human  Placenta.    X  260. 

Umbilical  Cord. 
The  umbihcal  cord  is  a  translucent  glistening  white  or  pearly  rope 
of  tissue  about  two  feet  in  length,  extending  from  the  umbihcus  to  the 
placenta.  It  consists  of  mucous  tissue  (p.  37)  covered  with  epithelium 
and  containing  at  birth  three  large  blood  vessels,  two  arteries  and  a  vein 
(Fig,  355,  B).  The  parallel  arteries  generally  wind  around  the  vein 
making  sometimes  forty  revolutions.  The  surface  of  the  cord  shows 
corresponding  spiral  markings  and  often  irregular  protuberances  called 


UMBILICAL   COED. 


309 


false  knots.     (True  Imots,   tied  by  the  intrauterine  movements  of  the 
embryo,  are  very  rare.)     There  are  no  lymphatic  vessels  or  capillaries 


--;  Decidual  cells. 


—  Sj-iicytium. 


Connective  tissue. 


Cell  knots. 


%\    O    ^% 


354. — From  a  Cross  Section  of  a  Mature  Human  Placenta.    X  260. 

in  the  cord  and  the  vessels  do  not  anastomose.     The  arteries  contain 
many  muscle  fibers  but  very  little  elastic  tissue  and  they  are  usually  found 


Fig.  355.— Cross  Sections  of  Umbilical  Cords.     A,  X  20,  from  an  embryo  of  2  mos. ;  B,  X  3,  at  birth. 
al.,  AUantois  ;  art.,  artery  ;  coe.,  coelom  ;  v.,  vein  ;  y.  s.,  yolk  stalk. 

collapsed  in  sections ;  their  contraction  is  of  interest  since  nerves  have  been 
traced  into  the  cord  for  only  a  very  short  distance.  The  vein  generally 
remains  open. 


3IO 


HISTOLOGY. 


The  umbilical  arteries  arise  within  the  embryo  as  the  principal  ter- 
minal branches  of  the  aorta;  parts  of  them  in  the  adult  are  called  the 
common  ihac  and  hypogastric  [internal  iliac]  arteries.  They  end  in  the 
capillaries  of  the  chorionic  villi.  The  single  umbilical  vein  is  due  to  a 
fusion  of  two;  within  the  body  only  the  left  remains,  passing  from  the 
umbihcus  along  the  under  surface  of  the  liver  (as  the  ductus  venosus) 
to  the  vena  cava  inferior. 

The  allantois  which  the  umbilical  vessels  accompany,  extends  the 
entire  length  of  the  cord  as  a  slender  tube  or  strand  of  cells.  At  birth  it  is 
rudimentary  but  may  be  found  usually  between  and  equidistant  from  the 
arteries.     It  is  more  conspicuous  when  Mallory's  stain  is  used.     Within 


Fig.  356. — Yolk  Sac  and  Persistent 
Vitelline  Vessels,  Exposed  by 
Reflecting  the  Amnion  at  the 
Distal  End  of  the  Cord. 
(Lonnberg.) 


Fig.  337. — Part  of  a  Human  Am- 
"niotic  Villus.     X  330. 

Ep.,  Epitrichium  ;  S.  C,  stratum  cor- 
neum  ;  S.  g.,  stratum  granulosum  ; 
S.  G.,  stratum  germinativum  ;  M.  B., 
homogeneous  layer;  F.  T.,  fibrous 
tissue  ;  A.  T.,  areolar  tissue. 


the  body  the  allantois  dilates  to  make  the  bladder,  and  if  its  prolongation 
into  the  cord  remains  tubular,  urine  may  escape  at  the  umbilicus  (through 
a  "urinary  fistula"). 

The  yolk  stalk,  surrounded  by  an  extension  of  the  body  cavity,  is 
found  in  young  umbihcal  cords  (Fig.  355,  A).  The  loop  of  intestine 
from  which  the  yolk  stalk  springs  may  also  extend  into  the  cavity  of  the 
cord,  and  if  it  has  not  been  drawn  into  the  abdomen  at  birth,  umbihcal 
hernia  results.  If  the  cavity  of  the  yolk  stalk  remains  pervious  the  intes- 
tinal contents  may  escape  at  the  umbihcus  (fecal  fistula).  Ordinarily  the 
stalk  and  its  vitelline  vessels,  together  with  the  coelom  of  the  cord,  have  been 
obliterated  before  birth  and  no  trace  of  them  remains  in  sections  of  the 
cord. 


VAGINA.  3  1 1 

The  yolk  sac  may  be  found  with  almost  every  placenta,  as  a  very  small  cyst 
adherent  to  the  amnion  in  the  placental  area.  If  the  distal  end  of  the  cord  is 
gently  stretched  a  wing-like  fold  appears  (Fig.  356),  differing  from  all  others  by 
containing  no  large  vessels ;  the  fold  indicates  the  direction  of  the  yolk  sac  which 
may  be  exposed  by  stripping  the  amnion  from  the  chorion.  It  may  be  beyond 
the  limits  of  the  placenta. 

Amniotic  villi  are  irregular,  flat,  opaque  spots  on  the  amnion  near  the  distal 
end  of  the  cord.  They  are  often  present  and  may  suggest  a  diseased  condition. 
As  seen  in  Fig.  357  they  are  areas  of  imperfectly  developed  skin;  since  epithelial 
elevations  occur  abundantly  over  the  cords  of  certain  mammals,  these  structures 
of  unknown  significance  are  probably  normal. 


Vagina  and  External  Genital  Organs. 

The  vagina  consists  of  a  mucosa,  (submucosa),  muscularis  and  fibrosa. 
Its  epithelium  is  thick  and  stratified,  its  outer  cells  being  squamous  and 
easily  detached.  It  rests  upon  the  papillae  of  the  tunica  propria,  and 
is  thrown  into  coarse  folds  or  rugae.  Glands  are  absent.  The  tunica 
propria  is  a  dehcate  connective  tissue  with  few  elastic  fibers,  containing 
a  variable  number  of  leucocytes.  Occasionally  there  are  soKtary  nodules, 
above  which  numerous  leucocytes  w^ander  into  the  epithehum.  The  sub- 
mucosa consists  of  strong  elastic  and  looser  white  fibers.  The  muscu- 
laris includes  an  inner  circular  and  a  small  outer  longitudinal  layer  of 
smooth  muscle.  The  fibrosa  is  a  firm  connective  tissue,  well  supplied 
with  elastic  elements.  Blood  and  lymphatic  vessels  are  found  in  the 
connective  tissue  layers,  and  wide  veins  form  a  close  network  between  the 
muscle  bundles.     There  is  a  ganghonated  plexus  of  nerves  in  the  fibrosa. 

The  mucous  membrane  of  the  vestibule  differs  from  that  of  the  vagina 
in  possessing  glands.  The  numerous  lesser  vestibular  glands,  0.5-3  ^^-  i^ 
diameter,  produce  mucus;  they  occur  chiefly  near  the  cHtoris  and  the 
outlet  of  the  urethra.  The  pair  of  large  vestibular  glands  [Bartholin's] 
also  produce  mucus;  they  correspond  with  the  bulbourethral  glands  in 
the  male  and  are  of  similar  structure.  The  hymen  consists  of  fine  fibered, 
vascular  connective  tissue  covered  with  mucous  membrane.  The  chtoris 
is  a  somewhat  erectile  body,  resembhng  the  penis.  It  includes  two  small 
corpora  cavernosa.  The  glans  cHtoridis  contains  a  thick  net  of  veins. 
It  is  not,  as  in  the  male,  at  the  tip  of  a  corpus  cavemosum  urethrae  which 
begins  as  a  median  bulb  in  the  perineal  region;  the  bulbus  in  the  female 
exists  as  a  pair  of  highly  vascular  bodies,  one  on  either  side  of  the  vesti- 
bule. Each  is  called  a  bulbus  vestibuli.  The  labia  minora  contain  seba- 
ceous glands,  0.2-2.0  mm.  in  size,  w^hich  are  not  connected  with  hair 
folhcles;  they  first  become  distinct  between  the  third  and  sixth  years. 
The  labia  majora  have  the  structure  of  skin. 


312 


HISTOLOGY. 


SKIN. 

The  skin  (cutis)  consists  of  an  ectodermal  epithelium,  the  epidermis, 
and  a  mesodermal  connective  tissue,  the  corium  (Fig.  358).  The  ecto- 
derm is  at  first  a  single  layer  but  soon  it  becomes  double,  the  outer  cells 

staining  more  deeply,  and 
being  notably  larger  than 
the  inner  cells.  Their  char- 
acteristic dome  shape  is 
seen  in  the  figure.  The 
outer  layer  has  been  named 
the  epitrichium  since  the  hairs  which  grow  up  through  the  under- 
lying epithelium  do  not  penetrate  it,  but  cause  it  to  be  cast  off.  The 
epitrichium  has  been  found  on  the  umbilical  cord,  and  in  places  on  the 
amnion.     It  may  possibly  be  related  with  the  chorionic  syncytium.     The 


Fig.  35S.— Skin  from  the  Occiput  of  ax  Embryo 

OF  2^  Months.     (After  Bowen.) 

Th^  outer  layer  of  dark  cells  is  the  epitrichium. 


Stratum  corneum 
Stratum  lucidum 

-     Stratum  granulosum 
Stratum  germinativum 


I.     Epi- 
I  dermis. 


Duct  of  a  sweat 
gland. 


*^/- 


Coil  of  a  sweat 
gland. 


Stratum 
papillare 


Corium 


Stratum 
reticulare  , 


Fat  tissue. 


Stratum  subcutaneum. 


Fig.  359.— Verticai 


V'  M  THE  Sole  of  the  Foot  of  an  Adult.  X  25. 


deeper  layer  of  ectoderm  becomes  stratified,  and  it  gives  rise  to  the  hairs, 
nails,  and  enamel  organs.  It  also  produces  two  types  of  glands,  the 
sebaceous  glands  which  are  usually  connected  with  hairs,  and  the  sweat 


SKIN. 


31: 


Epidermis. 


-  Corium. 


glands.  These  are  widely  distributed  through  the  skin;  locally  the  ecto- 
derm forms  the  mammary  glands,  ceruminous  glands  of  the  ear,  ciliary 
glands  of  the  eyehds,  and  other  special  forms.  The  greater  part  of  the 
surface    of   the   skin  presents 

many  little  furrows  which  in-  a  b  c  d 

tersect  so  that  they  bound 
rectangular  spaces.  On  the 
palms  and  soles  the  furrows 
are  parallel  for  considerable 
distances,  being  separated 
from  one  another  by  slender 
ridges  along  the  summits  of 
which  the  sweat  glands  open. 
The  ridges  are  most  highly 
developed  over  the  pads  of  tis- 
sue at  the  finger  tips  and  in 
the  interdigital  spaces  at  their 
bases.  Here  the  tactile  func- 
tion is  most  perfect.  The 
pads  are  very  prominent  in 
the    embryo   and    correspond 

with  the  "  walking  pads  "  of  camivora.    Similar  structures  occur  on  the  soles, 

Corium.     The  corium  is  a  layer  of  densely  interwoven  bundles  of 

connective  tissue  extending  from  the  epidermis  to  the  fatty,  areolar  sub- 

Depiessions  ■which 

, , =^.~' —  -^i?"^      were  occupied  by 

j-:--.-j-->  ^"x  "C^^^^^  papillae. 


Ridge  corresponding  to 
a  furrow  of  the  corium. 


Papillae 
of  A. 


Tactile 
corpuscle. 


Papillae 
of  D. 


Fig.  360. — Vertical  Section  from  the  Sole  of  the 
Foot  of  an  Adult,  showing  Four  Ridges  (A-D) 
with  a  Pair  of  Papillae  beneath  Each.  Between 
the  papillae  of  D  is  the  duct  of  a  sweat  gland.     X  25. 


Portion  of  the  duct  of 
a  sweat  gland. 


Fig.  361.— Epidermis  from  the  Skin  of  the  Dorsum  of  the  Human  Foot,  seen 
FROM  the  Lower  Surface.    X  120. 

cutaneous  tissue  (Fig.  359).  Its  epidermal  surface  exhibits  papillae  which 
are  tallest  and  most  numercfus  on  the  palms  and  soles.  Their  height  may 
be  0.2  mm.     In  the  skin  of  the  face  they  are  poorly  developed  and  in  old 


314 


HISTOLOGY. 


» 


Part  of  the  stratum 
corneum. 


Stratum  luciduni. 


Stratum 
granulosum. 


Stratum 
eerminativum. 


age  they  tend  to  disappear  entirely.  As  seen  in  Fig.  360  they  may  be 
definitely  arranged  beneath  the  ridges  of  the  finger  tips,  forming  a  double 
row  under  each;  the  grooves  between  the  ridges  correspond  with  epithe- 
lial depressions  between  the  papillae.  In  Fig.  361,  which  represents  the 
under  surface  of  the  epidermis,  the  relation  of  the  papillae  to  the  rect- 
angular markings  may  be  seen.  The  papillae  are  formed  of  tunica  propria, 
a  cellular  connective  tissue;  and  each  papilla  contains  terminal  capillary 
loops  or  a  tactile  corpuscle  (Fig.  126,  p.  105).    The  corpuscles  are  most 

numerous  in  the 
sensitive  finger  tips 
where  they  may 
occupy  one  papilla 
in  every  four. 

Beneath  the 
papillae  the  connec- 
tive tissue  bundles 
are  closely  inter- 
woven but  toward 
the  subcutaneous 
tissue  they  form  a 
coarse  network 
[hence  the  corium 
is  sometimes  divi- 
ded into  a  stratum 
papillare  and  a 
deeper  stratum  re- 
ticulare].  The  sub- 
cutaneous tissue  is 
areolar,  with  large 
areas  of  fat  cells; 
where  the  fat  forms  a  continuous  layer  it  is  called  the  pannicii- 
lus  adiposiis.  Columns  of  areolar  tissue  which  extend  to  the  hair 
follicles  and  glands  of  the  skin,  may  become  paths  for  infection  from 
the  surface  to  the  subcutaneous  tissue.  The  elastic  fibers  of  the  skin 
are  said  to  form  a  subepithelial  net,  a  thick  plexus  of  fine  fibers  beneath 
the  papillae,  and  layers  of  coarse  fibers  along  the  vessels  in  the  deeper 
part  of  the  corium  and  in  the  fascia.  The  subcutaneous  tissue  contains 
relatively  Kttle  elastic  tissue.  In  the  skin  of  the  face  and  joints,  elastic 
elements  are  most  abundant;  in  old  age,  throughout  the  skin,  they  decrease 
notably.  Smooth  muscle  fibers  constitute  the  arrector  muscles  of  the 
hairs;   as  a  membranous  layer  they  occur  only  in  the  tunica  dartos  of  the 


&%8 


^0        —  „^ 


?§' .      Tunica  propria  of 

^      -  the  corium. 

/      - 


5kV 


Fig.  362.— From  a  Section  through  the  Skin  of  the  Sole 
OF  the  Foot  of  an  Adult  M.\n.  x  360. 


SKIX.  315 

scrotum,  and  in  the  nipple.  Striated  muscle  fibers  in  the  skin  of  the  face 
represent  the  insertions  of  the  muscles  of  expression.  The  vessels  and 
nerves  of  the  corium  are  described  on  page  327. 

Epidermis.  The  epidermis  is  stratified  epithelium,  the  many  layers 
of  which  are  divisible  into  a  stratum  germinativum  and  a  stratum  corneum. 
The  former  includes  a  basal  row  of  columnar  cells  without  membranes, 
which  rest  on  the  papillae  of  the  corium.  Although  mitoses  are  seldom 
seen,  these  cells  multiply  and  produce  the  several  layers  of  polygonal  cells 
which  overlie  them.  The  latter  are  connected  by  numerous  slender  inter- 
cellular bridges,  as  seen  in  Fig.  31,  p.  30.  Because  of  this  striking  feature 
the  stratum  germinativum  was  formerly  called  the  stratum  spinosum 
[and  rete  Malpighii].  The  transition  to  the  stratum  corneum  or  outer 
layer  of  horny  flat  cells  is  quite  abrupt,  except  in  the  thick  skin  of  the 
palms  and  soles.  An.  incomplete  layer  of  coarsely  granular  cells  may 
interv'ene.  In  the  corneum  the  cells  acquire  a  horny  exoplasmic  mem- 
brane; the  bridges  become  short  stifl"  spines;  the  protoplasm  and  nucleus 
are  dried  and  shrunken  and  in  the  outermost  cells  the  nucleus  may 
wholly  disappear.  The  cells  become  flatter  toward  the  surface,  from 
which  they  are  constantly  being  desquaniated. 

The  process  of  cornification  presents  a  more  elaborate  picture  in 
sections  of  the  palms  and  soles.  Passing  outward  from  the  stratum  ger- 
minativum there  is  a  darkly  staining,  coarsely  granular  layer,  one  or  two 
cells  thick,  which  is  followed  by  a  clear  somewhat  refractive  band  in 
which  the  cell  outlines  are  indistinct.  This  layer  seems  saturated  with 
a  dense  fluid  formed  by  dissolution  of  the  underlying  granules.  In  haema- 
toxyline  and  eosine  specimens  the  granular  layer  or  stratum  granulosum  is 
followed  by  a  pink  and  then  by  a  bluish  band,  which  are  subdivisions  of 
the  clear  stratum  lucidum.  They  are  folloAved  by  a  thick  stratum  corneum. 
Except  in  the  palms  and  soles  the  granulosum  is  thin  and  the  lucidum  is 
absent.  Chemically  the  coarse  granules  of  the  stratum  granulosum  resemble 
keratin  (from  which,  they  differ  by  dissolving  in  caustic  potash) ;  they 
are  therefore  called  kerato-hyahn  granules.  Their  diffuse  product  in  the 
stratum  lucidum  is  named  eleidin.  In  the  corneum  it  becomes  pareleidin, 
which,  Kke  fat,  blackens  with  osmic  acid,  but  the  reaction  occurs  more 
slowly.  The  pareleidin  is  not  due  to  fat  entering  the  skin  from  oily  secre- 
tions on  its  outer  surface. 

The  color  of  the  skin  is  due  to  fine  pigment  granules  in  and  between 
the  lowest  layers  of  epidermal  ceUs;  a  few  smaUer  granules  occur  in  the 
corium.  Pigmented  connective  tissue  ceUs  are  found  near  the  anus, 
but  they  are  generally  infrequent  and  are  absent  from  the  palms  and 
soles.     The  possibiHty  of  the  mesenchymal  origin  of  epithehal  pigment 


3i6 


HISTOLOGY. 


is  stated  on  page  46.  It  is  probable  that  the  epidermal  pigment  arises 
in  the  cells  in  which  it  occurs.  The  origin  of  the  granules  found  between 
the  epithehal  cells  is  obscure. 

Nails. 
The  nails  are  areas  of  modified  skin  consisting  of  corium  and  epi- 
thelium.    The  corium  consists  of  fibrous  and  elastic  tissue,  the  bundles 


Corium 


Stratum 
germitiativiim. 


EpoiiVLliium. 


Fig.  363.— Dorsal  Half  of  a  Cross  Section  of  the  Third  Phai 
The  ridges  of  the  nail  bed  in  cross  section  appear  like 


ANX  of  a  Child.    X  i5- 
papillae. 


of  which  in  part  extend  vertically  from  the  periosteum  of  the  phalanx 
to  the  epithelium,  and  in  part  run  lengthwise  of  the  finger.  In  place  of 
papillae  the  corium  of  the  nail  forms  narrow  longitudinal  ridges  which 
are  low  near  the  root  of  the  nail  but  increase  in  height  toward  its  free 
distal  border;  there  they  abruptly  give  place  to  the  papillae  of  the  skin. 
At  the  proximal  end  or  root  of  the  nail  the  corium  has  tall  papillae. 

The  epithelium  consists  of  a  stratum  germinativum  and  a  stratum 
corneum,  but  the  latter  corresponds  with  a  thick  stra- 
tum lucidum.  In  the  embryo  the  horny  substance  is 
entirely  covered  by  a  looser  layer,  the  eponychium, 
and  this  name  is  applied  in  the  adult  to  the  skin-like 
tissue  which  overlaps  the  root  and  sides  of  the  nail 
(Fig.  363).  The  eponychium  is  the  stratum  corneum 
of  the  adjoining  skin.  Although  the  nail  cells  are 
formed  by  the  entire  underlying  stratum  germinati- 
vum, as  is  shown  by  the  increasing  thickness  of  the 
nail  toward  its  distal  edge,  yet  the  principal  production  is  at  its  proxi- 
mal root  beneath  the  crescentic  white  area,  the  lunula.  The  opacity  of 
the  nail  at  the  lunula  has  been  attributed  to  keratohyahn;  an  imper- 
fect stratum  granulosum  occurs  there.  The  pink  color  of  the  outer  por- 
tion is  due  to  blood  beneath,  which  is  seen  through  the  transparent  stra- 
tum lucidum.     The  cells  of  the   nail  may    be    separated  by   heating   to 


Fig.  364.— Cells  of  a 
Human  Nail.    X  240. 


HAIR. 


317 


Epidermis. 


Epithelial  column. 
/ 


C^  / 


boiling  a  fragment  placed  in  a  strong  solution  of  caustic  potash.  The 
cells  retain  their  nuclei  as  is  seen  in  Fig.  364.  The  for^Yard  movement 
of  the  nail  is  due  to  the  production  of  new  ceUs  from  behind. 

Hair. 
The  hairs  arise  as  local  thickenings  of  the  epidermis.  They  soon 
become  round  columns  of  ectodermal  cells  extending  downward  into  the 
corium  (Fig.  365).  x\s  the  columns  elongate  the  terminal  portion  becomes 
enlarged,  forming  the  bulb  of  the  hair,  and  a  mesodermic  papilla  occupies 
the  center  of  the  bulb.  On  that  side  of  the  epithehal  column  which  from 
its  obHquity  may  be  called  the  lowxr  surface,  there  are  found  two  swell- 
ings (Fig.  366  and  368).  The  outer  is  to  become  a  sebaceous  gland  dis- 
charging its  secretion  into  the  epithehal  column;  the  inner  or  deeper 
swelling  is  called  the  hair  matrix  and 
its  cells,  which  increase  by  mitosis- 
contribute  to  the  growth  of  the  col- 
umn. (The  lower  swelling  is  often 
described  as  the  place  of  insertion  of 
the  arrector  piH  muscle.)  Beginning 
near  the  bulbus  the  core  of  the  column 
separates  from  the  peripheral  cells; 
the  latter  become  the  outer  sheath  of 
the  hair.  The  core  forms  the  inner 
sheath  and  the  shaft  of  the  hair.  The 
cells  of  the  shaft  become  comified  just 
above  the  bulbus,  and  they  are  sur- 
rounded by  the   inner  sheath  as   far 

as  the  sebaceous  gland.  Beyond  this  point  the  inner  sheath  degen- 
erates so  that  in  later  stages  the  distal  part  of  the  shaft  is  imme, 
diately  surrounded  by  the  outer  sheath.  As  new  cells  are  added  to  the 
hair  from  below,  the  shaft  is  pushed  toward  the  surface.  The  central 
cells  in  the  outer  end  of  the  column  degenerate,  thus  producing  a  "hair 
canal"  which  is  prolonged  laterally  in  the  epidermis  (Fig.  369).  The 
shaft  enters  the  canal,  breaks  up  the  overl}dng  epitrichium,  and  projects 
from  the  surface  of  the  body  (Fig.  370).  That  portion  of  the  hair  which 
remains  beneath  the  epidermis  is  called  its  root.  In  addition. to  the  epi- 
thehal sheaths,  the  root  of  all  larger  hairs  possesses  a  connective  tissue 
sheath  derived  from  the  corium.  This  ser\'es  for  the  insertion  of  a  bimdle 
of  smooth  muscle  fibers  which  arise  in  connection  -^-ith  the  elastic  elements 
of  the  superficial  part  of  the  corium.  Since  this  muscle  by  contraction 
causes  the  hair  to  stand  on  end  it  is  called  the  arrector  pili.     Its  insertion 


Mesench  villa 


Mesenchvma. 


Fig.  365.  —  Vertical  Section  of  the  Skim 

OF    the     B.A.CK     OF   A     HUM.^N     FETUS     OF 

Five  Months.    X  230. 


Epidermis. 


Cells  of  the  hair  canal. 


>'^ 


Fig.  366. — Vertical  Section  of  the 
Skin  of  the  Glutaf.al  Region 
OF  A  Human  Fetus  of  Five 
Months.    X  230. 


Fig.  367. — Vertical  Section  of  the 
Skin  of  the  Back  of  a  Human- 
Fetus  OF  Five  and  a  Half 
Months.     X  230. 


Tangential  section  of  the  outer  sheath. 


Cornified  inner  sheath. 


Cell  nuclei  of  the  sheath  cuticle 

of  ; 

Inner     f  Huxley's  layer         i 

sheath.  ■)  „    a"d  of 

IHenle  s  layer. 


Ancctor  muscle. 


Hair  nialri.x. 


Hair. 

Fig.  368.— Vertical  Section  of  the  Skin  of  the  Forehead  of  a  Human- 
Fetus  of  Five  Months.    X  230.    Differentiation  of  the  sheaths  of  the  hair. 


HAIR. 


319 


Blood  vessel. 


Hair  matrix. 


Fig.  369. — ^Vertical  Section  of  the  Skin  of  the  Back  of  a 

Human  Fetus  of  Five  and  a  Half  Months.    X  120. 

The  staining  with  iron  haematoxylin  has  made  the  horny  parts  so 

black  their  details  are  invisible. 


is  always  below  the  sebaceous  gland  and  on  the  lower  surface  of  the  hair 

as    sho'WTi   in   Fig.    370. 

The   hairs   which   cover 

the  body  of  the  embrj'o 

and  which  to  a  variable 

extent  persist  after  birth, 

are    soft     and     downy; 

they  are  kno^^Ti  as  lan- 
ugo.     Arrector   muscles 

are  absent  from  the  lan- 
ugo of  the  nose,  cheeks 

and  Hps,  and  also  from 

the  eyelashes  (cilia)  and 

nasal  hairs  (vibrissae). 

In  describing  the  de- 
velopment of  hairs  it  has 

been  stated  that  a  hair 

consists  of  apapilla,  bulb, 

and  shaft;  and  that  the 

part  of  the  shaft  beneath 

the  epidermis  is  covered  with  a  connective  tissue  sheath,  an  outer  epithehal 

sheath,  and  below 
the  sebaceous 
gland,  with  an  in- 
ner epithelial 
sheath.  The  finer 
structure  of  the 
shaft  and  its 
sheaths  is  shoAMi  in 
the  cross  section. 
Fig.  371,  and  the 
longitudinal  s  e  c  - 
tion.  Fig.  372;  it  is 
described  in  the 
following  p  a  r  a  - 
graphs. 

The  connective 
tissue  sheath  is  de- 
rived from  the  co- 
rium.      It  is  found 

about  the  larger  hairs  where  it  may  be  divisible  into  three  layers.     The 


Shaft 


Sebaceous  gland 


Epithelial 
sheaths. 
Connective  tissue 
sheath. 


Papilla. 


Fat  cells. 
Fig.  370.— From  a  Thick  Sectiox  of  the  Human  Scalp. 


320  HISTOLOGY. 

outer  layer  is  a  loose  connective  tissue  with  longitudinal  bundles,  con- 
taining elastic  fibers  and  numerous  vessels  and  nerves.  The  middle  layer, 
w^hich  is  thicker,  consists  of  circular  bundles  of  connective  tissue  without 
elastic  fibers.  The  inner  layer  together  with  the  basement  membrane  of 
the  outer  epithelial  sheath  may  form  a  single,  transparent  hyaline  mem- 
brane. The  connective  tissue  portion  of  the  membrane  is  sometimes 
longitudinally  fibrous;  the  epithelial  part  is  homogeneous  and  provided 
with  small  pores.' 

The  outer  epithelial  sheath  is  an  inpocketing  of  the  epidermis.  The 
stratum  corneum  extends  to  the  sebaceous  gland;  the  stratum  granu- 
losum  continues  somewhat  deeper,  but  only  a  thinned  stratum  germina- 
tivum  can  be  followed  to  the  bulb. 


f    Longitudinal  fiber 
I  layer. 

Connective    J  Circular  fiber  laver. 
tissue  sheath.  | 

Ih     1-  h  ^'■^-        . 

t  Hyaline  membrane. — (L^j^. 

Outer  epithelial  sheath.  jLi^:^'  .'■';:'     '.    ,,," 

Henle's  layer.      \  r  '  ii')'il'© 

Inner  epithelial  J  '"  V  ,,  /' [      './lo^ 


sheath.  1  .\;'^^ 


Huxley's  layer. 


_JiV^ 


f  Sheath  and  hair  cutic-  _^+^.', 
Hair.  \        Cortical  substance.   ^"'^  0*" 


■^    if 


[  Medullary  substance. 


NN- 


■-.•  .-<d 


Fig.  371. — From  a  Horizontal  Section  of  the  Human  Scalp.    X  240. 
Cross  section  of  a  hair  and  its  sheaths  in  the  lower  half  of  the  root. 

The  inner  epithelial  sheath  extends  from  the  sebaceous  gland  to  the 
bulb.  It  begins  as  a  layer  of  cornified  cells  below  the  termination  of  the 
stratum  granulosum;  it  is,  however,  not  a  continuation  of  that  layer. 
Toward  the  bulb  the  inner  sheath  is  divisible  into  three  layers.  The 
outer  or  Henle's  layer  consists  of  one  or  two  rows  of  cells  with  occasional 
atrophic  nuclei;  for  the  most  part  they  are  non-nucleated.  The  middle 
or  Huxley's  layer  is  a  row  of  nucleated  cells,  and  the  inner  layer  or  cutic- 
ula  of  the  sheath  is  formed  of  non-nucleated  cornified  scales.  Toward 
the  bulb  both  the  cuticule  and  Henle's  layer  are  nucleated  and  the  three 
layers  become  indistinguishable  as  seen  in  Fig.  372.  Kerato-hyalin 
granules  which  occur  in  Huxley's  and  Henle's  layers  extend  nearer  the 
papilla  in  the  latter. 


HAIR. 


321 


Hair  cuticle.     Cortical  substance. 


I         I    ■     Medullary  substance.    ) 


>  Hair. 


.< 


Longitudinal  fiber 
layer. 


Circular  fiber  layer.    - 


-  ii%|. 


*  a 


Connective  tissue. 


Hyaline  membrane.    — 


Outer  epithelial 
sheath. 


Henle's  layer. 
Huxley's  layer 


Cuticle  of  the  inuei 
sheath. 


Papilla 


'^lt*ln    f>  '^i  I  r^  ^^'  -"^' 


--  ---^^^-S^^^^-e^^^o^J^^^eS^^^n^^e^n^^^^  ^^^^^°- 


322 


HISTOLOGY. 


The  shajt  of  the  hair  is  entirely  epitheHal.  Its  surface  is  covered  by 
a  thin  cuticula  which  is  formed  of  transparent  scales  directed  from  the  cen- 
ter of  the  shaft  outward  and  upward,  and  overlapping  like  shingles.  These 
are  non-nucleated  cornified  cells.  The  greater  portion  of  the  shaft  is 
included  in  the  cortex.  Toward  the  bulb  the  cortex  consists  of  soft  cells, 
but  distally  they  become  cornified,  elongated  and  compact;  their  nuclei 
are  then  linear.  Except  in  white  hairs  pigment  occurs  both  between  and 
in  these  cells.  Very  small  intercellular  air  spaces  are  found  in  the  cortex 
of  fully  developed  hairs.     The  medulla  when  present,  occupies  the  center 


Cortical  substance.    _  _L' 


Medullary  substance. 


Cuticle.    — 


Fig.  373. — Elements  of  a  Human  Hair  and  its  Sheath.     X  240. 
1,  White  hair  ;   2,  scales  of  the  cuticle  ;   3,  cells  of  the  cortical  substance  of  the  shaft ;  4,  cells  of  Huxley's 
layer;  5,  cells  of  Henle's  layer,  having  the  appearance  of  a   fenestrated  membrane;  6,  cells  of  the 
cortical  substance  of  the  root. 


of  the  shaft.  It  is  generally  a  double  row  of  cells  containing  kerato- 
hyalin  granules  and  degenerate  nuclei.  A  medulla  is  found  only  in 
large  hairs  and  it  terminates  before  reaching  their  tips. 

The  shedding  of  hairs.  Shortly  before  and  after  birth  there  is  a  gen- 
eral shedding  of  hair.  In  the  adult  the  loss  and  renewal  of  hairs  is  not 
periocHc  but  constant.  The  life  of  a  hair  in  the  scalp  may  last  1600  days. 
The  process  of  removal  begins  with  a  thickening  of  the  hyahne  membrane 
and  circular  fiber  sheath.  The  matrix  ceases  to  produce  the  inner  sheath 
and  consequently  the  cuticula  and  hair.  The  bulbus  becomes  cornified, 
forming  a  solid  frayed  end  of  the  shaft  as  seen  in  Figs.  375  and  376.     The 


HAIR, 


Z^3> 


A 
Sebaceous 


^-^P> 


B 
jlands. 


C 
Old  hair. 


D 
New  hair. 


increase  of  undifferentiated  cells  in  the  outer  sheath  and  matrix  forces 
the  degenerating  hair  with  its  inner  sheath  outward   TFig.  376;.     The 
comified  bulb  remains  near  the 
sebaceous  gland  at   the  outer 
hmit   of  the    matrix;   after   a 
variable    time    the   hair  falls 
out.    The  deep  portion  of  the    inner  sheath. 
outer  sheath,  emptied  of    its 
hair,    collapses  and  shortens, 
drawing  the  atrophic  papilla 
upward.    The  matrix  cells  pro- 
liferate causing   the   epithehal  375         376  Matrix. 

rr,rr\    tr>    rf^tnrn      tr,     itc    -former     F'G.  374.  —  FOUR  Stages  IN  the   Shedding  of  a  Hair, 
COra    LO    return     to     Itb   lOrmer  pr^m    a    section     of    the     Nasal    Skin    of    714 

depth  and  a  new  hair  develops        -  '"-•^ths    mbr\o.  Xdo. 

Jr  r  X,  beginning  of  the  new  hair. 

in  the  old  sheath.     This  hair 

in    growing    toward    the    surface     may     complete     the     expulsion     of 

its  predecessor. 


<^^^ 


Remains  of  inner 
sheath. 


^   Cornified 
%     ^\     .^        bulb. 


/ 
Matrix. 


Remains  of  inner 
sheath. 


Cornified 
bulb 


Thin  hyaline 
membrane. 


Epithelial  cord. 


Matrix. 


Thick  hyaline    ;  »  '"g  %%    ^       ^  ' 

membrane.  -     j^:   ^ij)  e^V^ 

Epithelial           -  >.  %     &    «  I.  Via 

cord. i^Jla^;-  5  ,  ^»^, 


Matrix  cells. 

Papilla.  - 


Fig.  375. — Lower  Part  of  Fig.  374,  A. 
/.  230. 


Atrophic  papilla. 


Connective  tissue. 


^U 


Fig.  376. — Lower  Part  of  Fig.  374,  B. 
X  230. 


324 


HISTOLOGY. 


Sebaceous  Glands. 

The  sebaceous  glands  are  simple,  branched  or  unbranched  alveolar 
structures  situated  in  the  superficial  layer  of  the  corium  and  usually  appen- 
ded to  the  sheath  of  a  hair  (Fig.  370).  In  connection  with  the  lanugo, 
a  large  gland  may  be  associated  with  a  very  small  hair  (Fig.  377),  and  in 
exceptional  cases  as  at  the  margin  of  the  lip  or  on  the  labia  minora,  they 
occur  independently  of  hairs.  They  vary  in  size  from  0.2  to  2.2  mm.,  the 
largest  being  found  in  the  skin  of  the  nose  where  the  ducts  are  macro- 
scopic.   None  are  found  in  the  palms  or  soles  where  hairs  also  are  absent. 

The  short  duct  is  a  prolongation  of  the  outer  sheath  of  the  hair  and  is 
formed  of  stratified  epithelium,  the  number  of  layers  of  which  decreases 
toward  the  alveoli.     The  alveoli  consist  of  small  cuboidal  basal  cells,  and 


Epidermis. 


Corium. 


Cell  with  shrunken 
nucleus. 


Oel!  with  well  devel- 
oped drops  of  se- 
cretion. 


Cell  with  developing 
drops  of  secretion. 


Cubical  cell. 


Fig.  377.— a.  From  .\  Vertical  Section  through  the  Ala  Nasi  of  a  Child.  X  4°-  C,  Stratum 
corneum  ;  M,  stratum  germinativum  ;  t,  sebaceous  gland  consisting  of  four  sacks,  a,  duct  of  the  same  ; 
W,  lanugo  hair,  about  to  be  shed,  h,  sheath  of  the  same,  at  the  base  of  which  a  new  hair,  x,is  forming. 

B,  From  a  Vertical  Section  of  the  Skin  of  the  Ala  Nasi  of  an  Infant.  X  240.  Sack  of  a 
sebaceous  gland  containing  gland  cells  in  various  stages  of  secretion. 


of  large  rounded  inner  cells  in  all  stages  of  fatty  metamorphosis.  As  the 
cell  becomes  full  of  vacuoles  the  nucleus  degenerates,  and  the  cell  is  cast 
off  with  its  contained  secretion.  This  in  fife  is  a  semi-fluid  material  com- 
posed of  fat  and  broken  down  cells. 

Glandulae  praeputiales  are  sebaceous  glands  without  hairs  which  are 
sometimes,  but  not  always,  found  on  the  glans  and  praeputium  penis. 
The  designation  "Tyson's  glands"  is  not  justified  since  Tyson  described 
the  epithehal  pockets  ^  to  i  cm.  long  which  regularly  occur  near  the  fren- 
ulum praeputii.  Praeputial  glands  and  crypts  are  not  found  in  the  em- 
bryo. The  praeputium  is  united  to  the  outer  surface  of  the  glans  by  an 
epithehal  mass,  which  often  persists  after  birth  and  is  broken  up  by  the 
formation  of  concentric  epithehal  pearls.  Glands  and  crypts  are  absent 
from  the  praeputium  and  glans  clitoridis. 


GLANDS    OF   THE    SKIN. 


525 


Sweat  Glands. 

The  glandulae  sudoriparae  are  long  unbranched  tubes  termina- 
ting in  a  simple  coil  (described  by  Oliver  Wendell  Holmes  as  resembling  a 
fairy's  intestine,  Fig.  378).  The  coil  is  found  in  the  deep  part  of  the  corium 
or  in  the  subcutaneous  tissue  (Fig.  359).  The  duct  pursues  a  straight 
or  somewhat  tortuous  course  to  the  epidermis 
which  it  enters  between  the  connective  tissue 
papillae.  Within  the  epidermis  its  spiral  wind- 
ings are  pronounced;  it  ends  in  a  pore  which  may 
be  detected  macroscopically. 

The  epithelium  of  the  ducts  consists  of  two  or 
three  layers  of  cuboidal  cells;  it  has  an  inner  cu- 
ticula,  and  an  outer  basement  membrane  covered 
by  longitudinal  connective  tissue  fibers.  With- 
in the  epidermis  its  walls  are  made  of  cells  of 
the  strata  through  which  it  passes.  The  secre- 
tory portion  of  the  gland  (3.0  mm.  long  according  to  Huber)  forms  about 
three-fourths  of  the  coil,  the  duct  constituting  the  remainder.  The  secretory 
epithelium  is  a  simple  layer  of  cells,  varying  from  low  cuboidal  to  columnar 
according  to  the  amount  of  secretion  which  they  contain.  Those  filled 
with  secretion  present  granules,  some  of  which  are  pigment  and  fat.  The 
product  is  eliminated  through  intra-  and  intercellular  secretory  capillaries. 


F1G.37S. — Model  OF  the  Coiled 
Part  of  a  Sweat  Gland 
FROM  THE  Sole  of  the 
Foot.     (After  Huber.) 


Menibrana  propria. 


Cuticula. 


Muscle  fibers. 


A.  Duct  in 
cross  section. 


n^ 


B.  Columnar  epithelium 
from  the  coiled  tubule. 


Nuclei  of 
jland  cells. 


Muscle 
fibers. 


-i^TT- 


C.  Surface  view 
of  the  coiled  tubule. 


D.  Low  epithelium  from 
a  coiled  tubule. 

Membrana  propria. 

Muscle  fibers. 

.f,^-'—  Muscle  nucleus. 
Cuticula. 

Membrana  propria. 

Muscle  fiber. 

E.  Cross  section  of 
coiled  tubule. 


Fig.  379. — A-D,  from  a  Section  of  the  Skin  of  the  Axilla;  E,  from  the  Finger  Tip  of  a  Man 
OF  23  Years.     X  230.    E  is  not  a  true  cross  section. 


It  is  ordinarily  a  fatty  fluid  for  oiling  the  skin,  but  it  becomes  the  watery 
sweat  under  the  influence  of  the  nerves.  The  gland  cells  are  not  destroyed 
by  either  form  of  activity.  The  secretory  tubule  is  surrounded  by  a 
distinct  basement  membrane,  within  which  there  is  a  row  of  small  longi- 
tudinally elongated  cells  described  as  muscle  fibers.     They  do  not  form 


326 


HISTOLOGY, 


a  complete  membrane,  and  they  appear  as  a  continuation  of  the  basal 
layer  of  cells  of  the  ducts. 

Sweat  glands  are  distributed  over  the  entire  skin  except  that  of  the 
glans  and  the  inner  layer  of  the  praeputium  penis.     They  are  most  numer- 


Epidermis. 


Corium. 


„    Branches  of  Ihe  subpapil- 
lary  arterial  network. 


— Veins  of  the  second  super- 
ficial plexus. 

\'eins  along  the  duct  of  a 
sweat  e:land. 


Subcutane- 
ous tissue. 


Large  vein. 


Vessel  to  the 
fat  tissue. 


Vessel  to  the 
sweat  eland. 


Fig.  380.— Part  of  a  Vertical  Sectio.s-  of  the  Injected  Skin  of  the  Sole  of  the  Foot.     X  20. 
The  \  cins  are  not  completely  filled  by  the  injection. 

ous  in  the  palms  and  soles.  In  the  axilla  there  are  large  forms  with  30 
mm.  of  coiled  tube.  They  acquire  their  large  size  at  puberty  and  have 
been  considered  as  sexual  "odoriferous"  glands.  In  the  vicinity  of  the 
anus  there  are  branched  sweat  glands,  and  large  unbranched  "circum- 
anal glands"  together  with  other  modified  forms. 


SKIN.  327 

Vessels  and  Nerves  of  the  Skin. 

The  arteries  proceed  from  a  network  above  the  fascia  and  branch  as 
they  ascend  toward  the  surface  of  the  skin.  Their  branches  anastomose, 
forming  a  horizontal  plexus  in  the  lower  portion  of  the  corium.  From 
this  plexus  branches  extend  to  the  lobules  of  fat  and  to  the  coils  of  the 
sweat  glands,  about  which  they  form  "baskets"  of  capillaries.  Other 
branches  pass  to  the  superficial  part  of  the  corium  where  they  again  anas- 
tomose before  sending  terminal  arteries  into  the  papillae.  The  super- 
ficial plexus  is  called  subpapillary,  and  from  it  the  branches  to  the  seba- 
ceous glands  and  hair  sheaths  are  derived.  The  papiUa  of  a  hair  receives 
an  independent  branch.  The  veins  which  receive  the  blood  from  the 
superficial  capillaries  form  a  plexus  immediately  beneath  the  papillae,  and 
sometimes  another  just  below  the  first  and  connected  with  it.  The  veins 
from  these  plexuses  accompany  the  arteries  and  the  ducts  of  the  sweat 
glands  to  the  deeper  part  of  the  corium,  where  they  branch  freely,  receiv- 
ing the  veins  from  the  fat  lobules  and  sweat  glands.  Larger  veins  con- 
tinue into  the  subcutaneous  tissue  where  the  main  channels  receive  specific 
names. 

The  lymphatic  vessels  form  a  fine  meshed  plexus  of  narrow  vessels 
beneath  the  subpapillary  network  of  blood  vessels.  It  empties  into  a 
wide  meshed  subcutaneous  plexus.  There  are  lymphatic  vessels  around 
the  hair  sheaths  and  both  sorts  of  glands. 

The  nerves  form  a  wide  meshed  plexus  in  the  deep  subcutaneous 
tissue,  and  secondary  plexuses  as  they  ascend  through  the  skin.  The 
sympathetic,  non-medullated  nerves  supply  the  numerous  vessels,  the 
arrector  pih  muscles,  and  the  sweat  glands;  an  epilamellar  plexus  out- 
side of  the  basement  membrane  sends  branches  through  the  membrane 
to  terminate  in  contact  with  the  gland  cells.  Medullated  sensory  nerves 
end  in  the  various  corpuscles  already  described  (page  105),  and  in  free 
terminations,  some  being  intraepithelial.  Medullated  fibers  to  the  hairs 
lose  their  myehn  and  form  elongated  free  endings  with  terminal  enlarge- 
ments in  contact  with  the  hyaline  membrane.  (The  nerves  to  the  tactile 
hairs  of  some  animals  penetrate  the  hyahne  membrane  and  terminate  in 
tactile  menisci  among  the  cells  of  the  outer  sheath.)  There  are  no  nerves 
in  the  hair  papilla.  The  corium  beneath  the  nails  is  rich  in  medullated 
nerves,  the  non-medullated  endings  of  which  enter  the  Golgi-Mazzoni 
type  of  lamellar  corpuscle  (having  a  large  core  and  few  lamellae),  or 
they  form  knots  which  are  without  capsules.  Elsewhere  the  skin  contains 
tactile  corpuscles  in  its  papillae  and  lamellar  corpuscles  in  the  subcutaneous 
tissue,  together  with  free  endings  in  the  corium  and  epidermis  (as  far 
out  as  the  stratum  granulosum). 


328  histology. 

Mammary  Glands. 
In  young  mammalian  embryos  generally,  the  mammary  glands  are 
first  indicated  by  a  thickened  hne  of  ectoderm  extending  from  the  axilla 
to  the  groin.  Later  much  of  the  line  disappears,  leaving  a  succession  of 
nodular  thickenings  corresponding  with  the  nipples.  In  some  mammals 
this  row  of  nipples  remains,  in  others  only  the  inguinal  thickenings, 
and  in  still  others  only  those  toward  the  axilla.  Thus  in  man  there  is 
normally  only  one  nipple  on  each  side.  In  an  embryo  of  25  cms.  (Fig. 
381)  several  soUd  cords  have  grown  out  from  the  ectodermal  proliferation. 
There  are  ultimately  from  15  to  20  of  these  in  each  breast  and  they  branch 
as  they  extend  through  the  connective  tissue.  At  birth  the  nipple  has 
become  everted,  making  an  elevation,  and  at  that  time  the  glands  in  either 
sex  may  discharge  a  little  milky  secretion  similar  to  the  colostrum  which 


Fig.  381.— Section  Through  thk  Mammary  Gland  of  an  Embryo  op  25  cms. 
1,  Connective  tissue  of  the  gland.     (After  Bascli,  from  McMurrich.) 

precedes  lactation.  The  glands  grow  in  both  sexes  until  puberty,  when 
those  in  the  male  atrophy  and  only  the  main  ducts  persist.  In  the  female 
enlarged  terminal  alveoh  are  scarcely  evident  until  pregnancy.  The 
glands  until  then  are  discoid  masses  of  connective  tissue  and  fat  cells, 
showing  in  sections  small  scattered  groups  of  duct-hke  tubes. 

Toward  the  end  of  pregnancy  each  of  the  15  or  20  branched  glands 
forms  a  mammary  lobe  and  its  alveolo- tubular  end  pieces  are  grouped  in 
lobules.  The  secretory  epithelium  is  a  simple  cuboidal  or  flattened  layer 
in  which  fat  accumulates  at  the  seventh  or  eighth  month.  It  first  appears 
as  granules  at  the  basal  end  of  the  cell,  where  it  is  received  in  combination 
from  the  surrounding  tissue.  This  fat  is  not  produced  by  the  gland  cell. 
The  lumen  of  the  alveoli  contains  leucocytes  which  have  passed  between 
the  epithelial  cells,  from  the  connective  tissue.  Some  of  them  degen- 
erate; others  receive  fat  from  the  gland  cells,  either  in  combination,  or  in 
drops  which  are  devoured  by  phagocytic  action.     The  fatty  leucocytes 


MAMMARY    GLAXDS.  3^9 

grow  to  considerable  size  and  are  called  colostrum  corpuscles.  Beneath 
the  alveolar  epithehum  there  are  basal  or  basket  cells  which  have  been 
compared  with  the  muscle  fibers  of  sweat  glands.  A  basement  membrane 
separates  them  from  the  connective  tissue  which  contains  many  mono- 
nuclear leucocytes  and  eosinophiUc  cells. 

After  the  birth  of  the  child  the  gland  cells  become  larger  and  are 
filled  with  stainable  secretory  granules  and  fat  droplets;  the  latter  are 
near  the  lumen  and  are  often  larger  than  the  nucleus  (Fig.  383;.  After 
two  days  of  lactation  some  of  the  gland  ceUs  are  flat  and  empty  of  secre- 
tion.    Others  are  tall  columnar,  with  a  rounded  border  toward  the  lumen; 

Branch  of  an  excretory  duct. 
«5r7-*^-^j....   ... 


r   %^.<;  •..-V»?->Jt.  ,    '. 


Connective  tissue. 

''''^B'tD 

--•>X:V>r'- Tubule. 

\''-  .         ''•':'":  .,.■■•;' 

'■^■r?: 

•\\j', ;,;/'; 0  ..;-•,;■-?■' 

■•r^'Vci^-  :.?r  /■'''■■.;• 

:\';"'.y-''M            .  Alveolo-tubular 
•■.,'■'/ 1-  :.•_.--■'          end  piece. 

Fig.  382. — Section  of  a  Hu.ma.v  Mam.marv  Gland  at  the  Period  of  Lactation.     X  50. 

often  they  contain  two  nuclei.  The  fat  within  them  is  not  a  degeneration 
as  in  sebaceous  glands,  nor  a  secretion  produced  by  the  nucleus;  it  is  a 
product  of  protoplasmic  activity,  and  may  fill  the  cell  several  times  before 
it  perishes.  Transitions  between  the  low  empty  cells  and  the  columnar 
forms  occur,  but  mitoses  are  absent  from  the  lactating  gland.  ^Mitotic 
divisions  are  numerous  during  pregnancy. 

Milk  consists  of  fat  droplets,  2-5  a  in  diameter,  floating  in  a  clear 
fluid  containing  nuclein  derived  from  degenerating  nuclei,  and  occasion- 
ally a  leucocyte  or  colostrum  corpuscle.  The  interstitial  connective  tissue, 
greatly  reduced  by  the  enlarged  glands,  also  contains  very  few  leucocytes 
and  eosinophilic  cells. 


33° 


HISTOLOGY, 


At  the  end  of  lactation  the  connective  tissue  increases  and  the  leuco- 
cytes reappear;  as  during  pregnancy,  they  form  colostrum  corpuscles. 
The  lobules  become  smaller  and  the  alveoli  begin  to  disappear.  In  old 
persons  all  the  end  pieces  and  lobules  have  gone  and  only  the  ducts  remain. 

The  ducts  are  lined  with  simple  columnar 
epithelium,  surrounded  by  a  basement  mem- 
brane and  generally  by  circular  connective  tis- 
sue bundles.  Toward  the  nipple  each  duct 
forms  a  considerable  spindle  shaped  dilatation, 
the  sinus  lactiferus.  The  epithelium  of  the 
outer  part  of  the  ducts  is  stratified  and  squa- 
mous. 

The  skin  of  the  nipple  and  of  the  areola 
at  its  base  contains  pigment  in  the  deepest 
layers  of  its  epidermis.  The  corium  forms  tall 
papillae  and  contains  smooth  muscle  fibers,  some  of  which  extend  vertically 
through  the  nipple  and  others  are  circularly  arranged  around  the  ducts. 
There  are  tactile  corpuscles  in  the  nipple,  which  becomes  rapidly  elevated 
upon  irritation,  due  to  muscular  rather  than  to  vascular  action.  There  are 
many  sweat  and  sebaceous  glands  in  the  areola  and  occasional  rudimen- 


y 


'm' 


Gland  cell. 


Membrana    Oil  drops, 
propria. 

Fig.  383.— From  a  Section  ok  the 
Mammary  Gland  of  a  Nurs- 
ing Woman.     X  250. 


Fig.  384. — A.,  Milk  Globules  from 
Hu.man  Milk.  X  560.  B.,  Ele- 
ments OF  the  Colostrum  of  a 
Pregnant  Woman.     X  560. 

I,  Cell  containing  uncolored  fat  globules  ; 
2,  cell  containing  minute  colored 
fat  globules;  3,  leucocyte;  4,  milk 
globules. 


■>,ff'p^mf^?'^ 


:^.. 


Fig.  385. — From  a  Thick  Section  of  the  Mam- 
mary Gland  of  a  Woman  Last  Pregnant  Two 
Vears  Before.     X  50. 

I,  Large  excretory  duct;  2,  small  excretory  duct;  3, 
gland  lobules,  separated  from  one  another  by 
connective  tissue. 


tary  hairs.  The  areolar  glands  [of  Montgomery]  are  branched  tubular 
glands  having  a  lactiferous  sinus  and  otherwise  resembling  the  constit- 
uent mammary  glands.  Their  funnel  shaped  outlets  are  surrounded  by 
large  sebaceous  glands.  The  areolar  glands  are  regarded  as  transitions 
between  sweat  glands  and  mammary  glands. 


SUPRARENAL    GLAXDS. 


331 


Blood  vessels  enter  the  breast  from  several  sources  and  form  capil- 
laries around  the  alveoli.  Lymphatic  vessels  are  found  in  the  areola, 
around  the  sinuses,  and  in  the  interlobular  tissue.  The  collecting  vessels 
pass  chiefly  toward  the  axilla;  a  few  penetrate  the  intercostal  spaces 
toward  the  sternum.     The  nerves  are  like  those  of  sweat  glands. 


SUPRARENAL  GLANDS. 

The  suprarenal  glands  are  two  flattened  masses  of  strands  of  cells, 
without  lumen  or  ducts,  situated  in  the  retroperitoneal  tissue  above  the 
kidneys.  The  right  is  generally  described  as  triangular  and  the  left  as 
crescentic.  They  are  between  one  and  two  inches  long,  not  quite  so  wide, 
and  about  a  quarter  of  an  inch  thick.  On  section  they 
present  macroscopically  a  yellowish  cortical  substance 
which  becomes  dark  brown  toward  the  center  of 
the  gland.  In  the  thicker  portions  there  is  a  vascu- 
lar medullary  substance  also  dark  colored,  related  to 
the  cortex  as  seen  in  Fig.  386.  In  many  lobes  the 
medulla  is  lacking  so  that  the  deep  portions  of  the 
cortex  of  the  two  sides  are  in  contact.  The  supra- 
renal glands  produce  a  secretion  received  by 
the  blood  (some  have  said  by  the  lymphatic  ves- 
sels also).  Death  follows  the  removal  of  the 
glands,  and  their  pathological  conditions  may  be 
fatal.  Intravenous  injection  of  suprarenal  extract 
causes  a  great  rise  in  blood  pressure. 

The  development  of  the  suprarenal  gland  indi- 
cates  a  radical  difference  between  the  cortex  and 

medulla.  In  the  sharks  these  components  form  separate  organs.  The 
interrenal  gland  which  corresponds  with  the  cortex,  consists  of  cords 
of  mesodermal  cells  and  has  apparently  a  sinusoidal  circulation. 
The  medulla  is  represented  by  a  peculiar  development  of  the  s}'m- 
pathetic  ganglia.  In  mammals  the  medulla  likewise  arises  by  the 
development  of  chromaffine  cells  in  relation  with  the  sympathetic  nerves. 
The  position  of  the  involved  nerves,  between  the  aorta  and  the  WoLfi&an 
body,  is  shown  in  Fig.  276,  C,  page  245.  The  sympathetic  portion  of  the 
gland  becomes  surrounded  by  dense  mesenchyma  in  which  the  cords  of  the 
cortex  are  differentiated.  Opinions  are  divided  as  to  whether  this  mesen- 
chyma is  derived  from  the  Wolffian  body  or  from,  the  coelomic  epithelium. 
As  the  kidneys  attain  their  permanent  position  the  suprarenal  glands  are 
found  above  them;  they  are  structurally  as  independent  of  the  kidneys  as 
are  the  liver  and  spleen. 


Cortex.  Medulla.  Vein. 

_  Fig.  386.— Section  of  the 
Suprarenal  Body  of 
A  Child.    X  15- 


332 


HISTOLOGY. 


The  cortical  substance  consists  of  cuboidal  cells  which  in  the  outermost 
zone  are  arranged  in  rounded  masses ;  in  the  middle  zone  they  form  cylin- 
drical columns;  and  in  the  deepest  layer  the  cords  unite  in  an  irregular 
network.  The  cortex  is  therefore  divided  into  a  zona  glomerulosa,  zona 
jasciculata  and  zona  reticularis  (Fig.  387).  The  cells  of  the  cortex  are 
about  15  //  in  diameter  and  contain  fat  droplets  causing  the  macroscopic 
yellow  appearance.  The  drops  are  especially  large  in  the  zona  fasciculata 
(Fig.  388),  and  are  small  or  even  absent  in  the  zona  reticularis.  The 
dark  brown  color  of  the  latter  is  due  to  pigment  which  becomes  con- 


Zona  glomerulosa. 


Zona  fasciculata. 


Zona  reticularis. 


Cell  cords  of  the 
medulla. 


Ner\'e  in  cross 
section. 


Ganglion  cells.    • 


Bundles  of  smooth  muscle 
fibers  in  cross  section. 


Veins.     -= 


Capsule. 


Cortex. 


Medulla. 


Fig.  3S7. — Section  of  a  Human  Suprarenal  Gland.     X  .so. 


spicuous  only  in  the  adult.  Besides  vacuoles  the  protoplasm  of  the  outer 
cells  contains  granules ;  the  nuclei  of  the  glomerular  zone  may  be  denser 
than  those  of  the  fascicular  layer.  The  cell  columns  are  in  close  relation 
with  the  endothehum  of  the  blood  vessels.  They  have  no  basement 
membrane,  and  are  separated  from  the  vessels  by  a  very  shght  amount 
of  reticular  tissue. 

The  medullary  substance  consists  of  chromaffine  cells  arranged  in 
elongated  strands  which  unite  and  form  a  network.  The  cells  are  very 
delicate  and  easily  become  stellate  by  shrinkage  even  in  well  fixed  prepara- 
tions.    They  have  round  nuclei  and  granular  protoplasm  but  their  specific 


SUPRARENAL    GLANDS. 


333 


Cells  of 
the  zona 

fasciculata 
with  fat 
vacuoles. 

Connective 
tissue. 


/  a- 


'^^- 


Pigmented 

cells. 
Cells  of  the 

zona 
reticularis. 


Cell  of  the 
medulla. 


Fig. 


From  a  Section  of  the  Scprarekal 
Gland  of  an  Adult.  X  360. 


characteristic  is  an  affinity  for  chromic  acid  or  chromium  saks,  by  which 

they  are  colored  brown.     Similar 

cells  occur  in  some  sympathetic 

ganglia  and  in  the  glomus  carot- 

icum. 

The  capsule  of  the  supra- 
renal glands  is  connective  tis- 
sue, said  to  contain  smooth  mus- 
cle fibers,  blood  and  lymphatic 
vessels,  nerves  and  small  gang- 
lia. It  sends  prolongations  into 
the  interior.  Elastic  fibers  are 
found  in  the  medulla  but  they 
are  very  few  or  absent  in  the 
cortex. 

The  arteries  divide  in  the 
capsule  into  many  small  branches 
which  penetrate  the  cortex  and 
there  form  a  long-meshed  cap- 
illary network;  toward  and 
within  the  meduUa  the  meshes  become  round.  Some  arteries  pass 
directly   from   the    capsule   to    the    medulla,    without    branching   in   the 

cortex.  The  larger  of 
the  numerous  veins  which 
arise  in  the  medulla  are 
accompanied  by  longitu- 
dinal bundles  of  smooth 
muscle  fibers.  Before 
leaving  at  the  hilus  they 
unite  to  form  the  supra- 
renal vein.  Lymphatic 
vessels  have  been  record- 
ed in  the  capsule  and 
medulla. 

The  numerous 
mostly  non  -  medullated 
nerves,  of  which  a 
human  suprarenal  gland 
receives  about  thirty 
small  bundles,  proceed 
chiefly  from  the  coehac  plexus  and  pass  with  the  arteries  from  the  capsule 


Artery. 


Ci^. 


s,m 


Long  meshed 
capillary  net 
of  the  cortex. 


Fig 


Round  meshed 
net  of  the 
medulla. 


I.— From  an  Injected  Section  of   the   Suprarenal 
Gland  of  a  Child,     x  50. 


334  HISTOLOGY. 

into  the  medulla.  Branches  from  the  plexus  in  the  capsule  descend  between 
the  cell  groups  of  the  cortex  and  terminate  on  the  surface  of  the  cells  in  the 
two  outer  zones;  they  do  not  enter  between  the  separate  cells.  The  plexus 
in  the  zona  reticularis  is  more  abundant,  but  here  also  only  groups  of  cells 
are  supphed.  In  the  medulla  the  nerves  are  extraordinarily  abundant  and 
each  cell  is  surrounded  by  fibers.  Groups  of  sympathetic  ganglion  cells 
may  be  found,  but  these  rarely  occur  in  the  cortex.  A  part  of  the  nerves 
terminate  in  the  walls  of  the  vessels. 

In  the  vicinity  of  the  ductus  deferens  and  in  the  broad  Hgament  of  the 
uterus,  suprarenal  bodies  may  occur,  consisting  only  of  cortical  substance. 
Groups  of  chromafhne  cells  have  been  found  in  relation  with  the  paro- 
ophoron and  paradidymis. 

BRAIN  AND  SENSE  ORGANS. 
Br.\in. 

DE\T:LOPiIEXT   AXD    GENERAL    AnATOMY. 

In  a  previous  section  the  formation  of  the  medullary  tube  from  the 
primitive  ectoderm  has  been  described,  and  it  has  been  stated  that  the 
posterior  portion  of  the  tube  becomes  the  spinal  cord  and  that  the  anterior 
portion  forms  the  brain.  In  a  human  embryo  of  4.0  mm.,  the  tube  still 
opens  freely  through  a  large  anterior  neuropore,  the  extent  of  its  connection 
with  the  epidermal  ectoderm  being  indicated  in  Fig.  390,  A.  The  tube 
has  become  bent  in  two  places;  the  posterior  or  neck  bend  is  near  the 
junction  of  the  cord  and  brain,  the  line  of  separation  between  which  must 
be  arbitrarily  drawn  both  in  the  embryo  and  in  the  adult;  the  anterior  or 
head  bend  occurs  in  a  part  of  the  tube  called  the  mid-brain  (mesencephalon). 
In  front  of  the  mid-brain  is  the  fore-brain  (prosencephalon)  and  behind  it  is 
the  hiiid-brain  (rhombencephalon) .  The  entire  brain  is  therefore  divided 
into  fore-brain,  mid-brain,  and  hind-brain.  In  an  early  stage  the  fore- 
brain  produces  two  lateral  outpocketings,  one  on  either  side,  called  the 
opiic  vesicles.  Each  expands  distally  to  form  the  retina  of  an  eye  and  its 
connection  with  the  fore-brain  becomes  reduced  to  a  slender  stalk.  In 
later  stages  the  depression  on  the  inner  wall  of  the  brain  which  marks  the 
position  of  the  stalk  is  called  the  optic  recess. 

The  hind-brain  soon  becomes  rhomboid  or  kite-shaped  as  seen  from 
its  dorsal  surface.  This  is  due  to  a  widening  of  the  cavity  of  the  medullary 
tube;  its  lateral  walls  spread  apart  and  the  roof  plate  becomes  thin  and 
transparent.  The  dilated  cavity  of  the  hind-brain  is  called  the  fourth 
ventricle;  the  cavity  of  the  mid-brain  in  the  adult  is  a  slender  passage 
called  the  aqueduct  [of  Sylvius];    it  becomes  vertically  expanded  in  the 


DEVELOPMENT    OF   THE    BRAIN. 


335 


fore-brain  to  form  the  third  ventricle.  These  two  ventricles  and  the 
aqueduct  are  continuous  with  the  central  canal  of  the  spinal  cord  and 
represent  the  original  cavity  of  the  medullary  tube. 

In  an  embryo  of  lo  mm.  (Fig.  390,  B)  the  hind-brain  maybe  subdi- 
vided into  the  myelencephalon  posteriorly  and  the  metencephalon  anteriorly. 
The  constriction  between  the  hind-brain  and  mid-brain  is  called  the 
isthmus.  The  mesencephalon  remains  undivided;  the  fore-brain  is  repre- 
sented by  the  diencephalon  posteriorly  and  the  telencephalon  anteriorly. 
Thus  there  are  six  funda- 
mental subdivisions  of  the 
brain.  Their  further  de- 
velopment is  illustrated  in 
the  median  sagittal  sections 
of  the  brain,  Figs.  392  and 
393,  and  may  be  briefly  de- 
scribed as  follows. 

The  myelencephalon 
becomes  the  medulla  oblon- 
gata. It  transmits  the 
fibers  passing  between  the 
cord  and  the  brain;  it  re- 
ceives the  sensory  roots  of 
the  vagus  and  glossopharyn- 
geal nerves  and  contains 
the  groups  of  cell  bodies 
from  which  their  lateral 
roots  arise  [the  lateral  root 
of  the  vagus  being  called 
the  accessory  nerve].  It 
also  contains  the  cell  bodies 
from  which  arise  the  ven- 
tral roots  which  make  the 

hypoglossal  nerve.  (These  nerves  are  shown  in  Fig.  113,  p.  96,  and  in 
Fig.  391,  B.)  The  medulla  also  includes  groups  of  cell  bodies,  the  pro- 
cesses of  which  do  not  leave  the  central  nervous  system.  Such  groups 
are  called  nuclei;  the  gray  substance  in  most  of  the  ventral  portion  of 
the  brain  is  in  the  form  of  separate  nuclei  and  not  in  continuous  columns 
as  in  the  cord. 

The  metencephalon  produces  the  pons  ventrally  and  the  cerebellum 
dorsally.  The  pons  receives  the  sensory  roots  of  the  trigeminal,  inter- 
mediate and  acoustic  nerves;   it  gives  rise  to  the  lateral  roots  of  the  tri- 


FiG.  390. — A,  The  Brain  of  a  4.0  mm.  Human  Embryo  (after 
Bremer);  B,  the  Brain  of  a  10.2  mm.  Embryo  (after  His). 

Except  the  isthmus,  is.,  the  principal  subdivisions  of  the  brain 
are  indicated  by  prefixes  of  the  term  encephalon\  sp.C, 
spinal  cord  ;  h.,  hemisphere  ;  0.  v.,  optic  vesicle  ;  r.,  rhinen- 
cephalon  ;  v.,  roof  of  the  fourth  ventricle. 


33^ 


HISTOLOGY. 


geminus  and  intermedius  (facial)  and  to  the  ventral  root  which  makes  the 
abducens.  The  pons  transmits  the  ascending  and  descending  fibers 
between  the  cord  and  the  anterior  portion  of  the  brain,  together  with 
fibers  to  and  from  the  medulla.  Many  fibers  of  the  pons  pass  through  the 
lateral  wall  of  the  brain-tube  into  the  cerebellum,  forming  a  large  bundle 
on  each  side,  called  the  brachium  pontis  (Fig.  391).  The  cerebellum  also 
receives  on  each  side  a  bundle  from  the  anterior  part  of  the  brain,  the 
brachium  conjunciivum,  and  another  from  the  medulla,  the  restijorm  body. 
These  three  bundles  not  only  contain  fibers  to  the  cerebellum  but  also 
those  passing  from  it.     The  cerebellum  (Fig.  393)  is  a  large  lobular  mass 


^\ 

^abd 

•0 

^-^ 

/ 

"-.  '/' 

-int. 

^^ 

ac. 

Y-g\ 

3. 

-  >a . 

ace. 

liv 

Fig.  391. — A,  Dors.m.  .and  B,  Ventral  Vikw  of  the  Postkrior  P.art  of  the  Adult  Br.ain.    The 
Cerebellum  and  Roof  of  the  Fourth  Ventricle  has  been  Removed  from  A. 

b.  c,  Brachium  conjunctivum  ;  b.  p.,  brachium  pontis  ;  c.  m.,  corpus  mamillare ;  c.  p.,  cerebral  peduncle ; 
C.  q.  a.  and  c.  q.  p.,  anterior  and  posterior  corpora  quadrigemina ;  inf.,  infundibuliim  ;  med.,  medulla  ; 
ol.,  olive;  p.,  pons;  p.  b.,  pineal  body  ;  pyr.,  pyramid  ;  r.  b.,  resliform  body  ;  ven.,  floor  of  fourth  ven- 
tricle. The  nerves  are — oc,  oculomotor  ;  tr.,  trochlear  ;  tri.,  trigeminal  ;  abd.,  al^ducens  ;  int.,  inter- 
medius, fa.,  its  facial  portion;  ac,  acoustic  ;  gl.,  glossopharyngeal;  va.,  vagus,  acc,  its  accessory 
portion;  by.,  hypoglossal. 


of  nerve  tissue,  consisting  of  an  arborizing  medulla  of  white  substance,  and 
a  cortex  composed  of  special  forms  of  nerve  cells. 

The  isthmus  presents  on  its  dorso-lateral  surfaces  the  brachia 
conjunctiva.  Beneath  the  floor  of  the  central  cavity  or  aqueduct  it 
contains  the  motor  cells  from  which  the  fibers  of  the  troch- 
lear nerve  arise.  After  crossing  to  the  opposite  side  above  the  aqueduct, 
these  fibers  emerge  from  the  dorsal  surface  of  the  isthmus.  Ventrally 
the  tracts  of  fibers  extending  between  the  hind -brain  and  the  fore -brain 
form  projecting  elevations  which  diverge  as  they  pass  forward;  the  eleva- 
tions are  called  the  peduncles  of  the  cerebrum. 

The  mesencephalon  forms  dorsally  four  rounded  elevations,  the 
corpora  quadrigemina.     The  superior  or  anterior  pair  receives  fibers  fomr 


DEVELOPMENT   OF   THE    BRAIN. 


337 


Fig.  392.— Sagittal  Section  of  the  Brain  of  a  Three  Months  Embryo.     (After  His.) 

cbl.,  Cerebellum;  hem.,  hemisphere;  hy.,  hypophysis  (posterior  lobe);  isth.,  isthmus;  med.:  med 
ulla  oblongata ;  mes.,  mesencephalon;  ol.  b.,  olfactory  bulb;  0.  r.,  optic  recess;  p., pons;  p.  b 
pineal  body;  p.  s.,  pars  subthalamica  ;  th.,  thalamus. 


Fig.  393.— Median  Sagittal  Section  of  an  Adult  Brain. 
cbl.,  Cerebellum;  c.  C,  corpus  callosum  ;   c.  q.,  corpora  quadrigemina  ;   hy.,  posterior  lobe  of  the  hypoph- 
ysis ;   med.,  medulla  oblongata  ;   0.  b.,  olfactory  bulb  ;   0.  r.,  optic  recess  ;  p.,  pons;  p.  b.,  pineal  body  ; 
p.  S.,  pars  subthalamica  ;  th.,  thalamus. 


338  HISTOLOGY. 

the  optic  tract,  and  gives  rise  to  some  which  connect  with  the  motor  cells 
of  the  nerves  to  the  eye  muscles;  others  pass  down  the  spinal  cord  close 
beside  the  median  ventral  fissure.  Thus  the  anterior  corpora  are  centers 
of  optic  reflexes.  The  posterior  or  inferior  corpora,  which  are  smaller,  are 
in  relation  through  an  intervening  group  of  neurones,  with  the  acoustic 
nerves;  thus  they  are  centers  of  auditory  reflexes.  The  mesencephalon 
gives  rise  to  the  ventral  root  which  forms  the  oculomotor  nerve.  The 
cerebral  peduncles  which  begin  in  the  isthmus  extend  under  the  mesen- 
cephalon. 

The  diencephalon  has  on  its  median  dorsal  surface  the  pineal  body 
[epiphysis].  This  is  a  small  nodular  structure  which  is  thought  to  represent 
a  rudimentary  median  eye,  such  as  is  more  clearly  indicated  in  reptiles. 
The  upper  part  of  the  lateral  walls  of  the  diencephalon  are  each  thickened 
by  a  mass  of  nerve  tissue  called  the  thalamus.  The  thalami  of  the  two 
sides  bulge  inward  so  that  their  most  prominent  parts  adhere  across  the 
third  ventricle.  Fibers  from  the  retina  connect  with  nerve  cells  in  the 
thalamus,  the  latter  sending  their  processes  to  the  hemispheres;  thus  the 
thalami  have  an  important  relation  with  the  optic  tracts.  The  walls  of  the 
diencephalon  below  the  thalamus  form  the  pars  mamillaris  hypothalami. 
This  part  of  the  hypothalamus  includes  the  two  mamillary  bodies  found 
side  by  side  on  the  ventral  wall  of  the  diencephalon  (Fig.  391,  B). 

Telencephalon.  The  fibers  from  the  posterior  part  of  the  brain  pass 
outside  of  the  thalami  to  terminate  in  the  dorso-lateral  walls  of  the  telen- 
cephalon. As  seen  in  Fig.  390,  B,  this  part  of  the  fore-brain  forms  a 
hemispherical  outpocketing  on  either  side,  into  each  of  which  a  pro- 
longation of  the  third  ventricle  extends ;  the  extensions  are  called  lateral 
-ventricles  (and  are  counted  as  the  first  two).  The  hemispheres  enlarge, 
growing  back  so  as  to  cover  the  posterior  portion  of  the  brain.  Their  walls, 
which  externally  are  subdivided  by  grooves  into  convolutions,  constitute 
the  pallium  of  the  hemispheres.  The  olfactory  bulb  is  the  expanded 
termination  of  the  part  of  the  hemispheres  which  receives  the  olfactory 
nerves.  The  entire  olfactory  tract  is  called  the  rhinencephalon.  The 
corpus  striatum  is  a  deep  portion  of  the  hemisphere  found  outside  of  the 
thalamus;  anteriorly  it  forms  the  outer  wall  of  the  beginning  of  the 
lateral  ventricle.  The  hemispheres  are  connected  with  one  another  by  a 
great  transverse  commissure,  the  corpus  callosum,  through  which  fibers 
pass  from  one  to  the  other.  The  principal  subdivisions  of  the  hemisphere 
are  therefore  the  pallium,  rhinencephalon,  corpus  striatum  and  corpus 
callosum. 

Besides  the  hemispheres,  the  telencephalon  forms  the  pars  optica 
hypothalami.     This  includes  the  optic  recess  in  front  on  either  side,  and 


DEVELOPMENT    OF    THE    BRAIN. 


339 


the  injundihulum  in  the  mid- ventral  Hne.  The  infundibulum  terminates 
in  an  expansion  which  is  the  posterior  lobe  of  the  hypophysis.  This 
body,  together  with  the  anterior  lobe  derived  from  the  oral  ectoderm  but 
later  severed  from  it,  is  lodged  in  the  sella  turcica  of  the  sphenoid  bone. 
The  development  of  the  brain  is  summarized  in  the  following  table 
(after  His). 

f  Myelencephalon Medulla  oblongata. 

"'"''■'"■*'" j  Metencephalon {  ^"l^.^b.n^^. 

[  Isthmus Isthmus. 

Mid-brain Mesencephalon 1  Cerebral  pedunc]es._ 

^  i  Corpora  quadrigemma. 

f  Mamillary  part  of  the  hypothal- 

f  Diencephalon i  ^u^?^^* 

I  -^  Inalamus. 

I  [  Pineal  body. 

Fore-brain !  [  Optic  part  of  the  hypothalamus. 

Hypophysis  (posterior  lobe). 

I  Hemisphere: 

[  Telencephalon -j  Pallium. 

I  Rhinencephalon. 

I  Corpus  striatum. 

^  Corpus  callosum. 

Medulla  Oblongata. 

Before  considering  the  medulla  the  student  should  review  the  arrange- 
ment of  fiber  tracts  in  the  spinal  cord  (Fig.  147,  p.  121).  The  cerebro- 
spinal fasciculi,  both  ventral  and  lateral,  consist  of  the  fibers  which 
descend  from  the  hemispheres.  These  four  fascicuK  of  the  cord  arise 
from  two  in  the  medulla,  which  there  produce  a  pair  of  ventral  swellings 
(pyramids)  shown  in  Fig.  391,  B.  In  the  section.  Fig.  395,  it  is  seen  that 
the  pyramids  are  in  the  position  of  the  ventral  cerebro-spinal  tracts  of  the 
cord.  In  the  lower  or  posterior  part  of  the  medulla  the  greater  number 
of  fibers  in  each  pyramid  crosses  through  the  ventral  commissure  to  the 
opposite  side;  thence  they  proceed  across  the  gray  substance  to  the 
lateral  cerebro-spinal  fasciculus,  which  they  form  (Fig.  394).  The  crossing 
is  called  the  decussation  of  the  pyramids,  or,  since  these  fibers  terminate 
about  motor  cells,  it  is  called  the  motor  decussation.  The  relatively 
small  number  of  pyramidal  fibers  which  do  not  decussate  in  the  medulla, 
form  the  ventral  cerebro-spinal  fascicuH  of  the  spinal  cord. 

The  fibers  from  the  spinal  gangHa  ascend  to  the  medulla  in  the  cuneate 
and  gracile  fascicuK.  Within  the  medulla  their  fibers  terminate,  but 
their  course  toward  the  hemispheres  is  prolonged  by  a  second  group  or 
"  relay"  of  nerve  cells,  the  bodies  of  which  form  four  nuclei.     These  nuclei 


340 


HISTOLOGY. 


appear  as  additional  columns  or  horns  on  the  dorsal  part  of  the  gray  H 
(Fig.  395);  the  inner  pair  are  the  nuclei  0}  the  gracile  fasciculus,  and  the 
outer  ones  are  nuclei  0}  the  cuneate  fasciculus.  In  them  the  fibers  from 
the  cord  terminate  and  others  arise  which  cross  beneath  the  central  canal 
to  the  opposite  side  of  the  medulla  (Fig.  395).  Then  they  pass  forward 
in  right  and  left  bundles  known  as  lemnisci  or  fillets.  The  decussation  0} 
the  lemnisci  occurs  higher  up  in  the  medulla  (that  is,  more  anteriorly)  than 
that  of  the  pyramids;  and,  after  crossing,  the  fillets  remain  internal  to  the 
pyramids. 

With  the  sensory  and  motor  decussations  the  resemblance  between 


dcr; 


f.c.v. 


Fig.  394.— Section  of  the  Cord  at  the 
Level  of   the  First  Cervical  Nerve. 

The  right  half  of  the  section  shows  the  effect 
of  Weigert's  stain,  the  myelinated  portions 
being  dark ;  the  left  half  shows  the  gray 
substance  stippled  and  the  white  is  blank. 
f.  C,  Fasciculus  cuneatus;  f.  c.  I.,  fascicu- 
lus cerebro-spinalis  lateralis  ;  f.  c.  v.,  fascic- 
ulus cerebro-spinalis  ventralis  ;  f.  g.,  fascic- 
ulus gracilis;  d.  C,  dorsal  column;  d.  p., 
decussation  of  the  pyramids;  d.  r.,  dorsal 
root  of  first  cervical  nerve;  v.  c,  ventral 
column. 


t.s.n.t 


Fig.  395.— Section  of  the  Medulla.  (After 
Dejerine.) 

d.  C,  Dorsal  column;  d.  I.,  decussation  of  the 
lemnisci;  f.  c,  fasciculus  cuneatus;  n.  acc, 
nucleus  of  the  accessory  portion  of  the  vagus  ; 
n.  C,  cuneate  nucleus  ;  n.  g.,  gracile  nucleus  ; 
py.,  pyramid  ;  t.  S.  n.  t.,  spinal  tract  of  the  tri- 
geminal nerve  ;  v.  c,  ventral  column. 


the  medulla  and  spinal  cord  is  lost.  The  gray  substance  no  longer  forms 
an  H,  and  the  dorsal  fiber  tracts  have  become  ventral;  the  central  canal 
expands  to  make  the  fourth  ventricle,  as  seen  in  Fig.  396.  The  lemnisci 
form  vertical  bands  of  white  substance  on  either  side  of  the  median  ventral 
raphe.  The  pyramids  cause  protrusions  of  the  ventral  surface.  Dorsal 
to  each  there  is  a  large  nucleus,  the  olive,  which  also  makes  an  external 
elevation  (Fig.  391,  B).  Its  gray  substance  forms  a  convoluted  capsule; 
it  receives  fibers  from  the  cord  and  cerebellum,  and  gives  rise  to  some 
which  cross  through  the  median  raphe  and  ascend  to  the  cerebellum  in  the 
restiform  body.  The  restijorm  body,  which  forms  the  dorso-lateral  portion 
of  sections  of  the  upper  part  of  the  medulla,  contains  olivary  fibers,  those 


MEDULLA   OBLONGATA. 


341 


t.s.  n.h.  V. 


of  the  cerebello-spinal  fasciculus  of  the  cord,  some  from  the  gracile  and 
cuneate  nuclei,  and  some  from  other  nuclei  in  relation  with  the  sensory 
roots  of  the  cranial  nerves. 

The  cerebral  nerves  of  the  medulla  (and  pons  also)  are  arranged  in 
general  as  follows.  The  ventral  roots  arise  from  groups  of  cell  bodies, — 
the  nuclei  of  the  nerves,  situated  beneath  the  floor  of  the  ventricle  near  the 
median  hne.  The  nucleus  of  the  hypoglossal  nerve  is  seen  in  Fig.  396. 
The  lateral  roots  arise  from  nuclei  more  deeply  placed  and  further  from 
the  median  line;  their  fibers  may  pass  upward  and  inward  toward  the 
ventricle  before  turning  downward  and  outward  to  leave  the  brain.  The 
nucleus  ambiguus  (Fig.  396)  gives  rise  to  the  lateral  roots  of  the  vagus  and 
glossopharyngeus.  Like 
the  motor  cells  of  the 
spinal  cord,  those  of  the 
brain  are  also  in  connection 
with  descending  fibers  of  the 
pyramidal  tract.  The  dor- 
sal roots  on  entering  the 
brain  generally  divide  into 
a  short  ascending  branch 
and  a  longer  descending 
one.  The  tractus  solitari- 
us  (Fig.  396)  contains  the 
descending  fibers  of  the 
vagus  and  glossopharyn- 
geus; the  large  spinal  tract 
composed  of  the  descend- 
ing fibers  of  the  trigeminus 
is  shown  in  Figs.   395  and 

396.  The  dorsal  root  fibers  end  in  nuclei  corresponding  with  the 
gracile  and  cuneate  nuclei  of  spinal  nerves.  Fibers  from  the  internal 
nuclei  of  the  cerebral  sensory  nerves  join  the  lemniscus  and  proceed 
toward  the  hemispheres. 

Pons. 

The  ventral  swelling  characteristic  of  the  pons  is  due  to  the  transverse 
fibers  of  the  brachium  pontis  (Fig.  391).  These  cross  beneath  and  through 
the  pyramidal  bundles.  Some  of  them  arise  from  numerous  groups  of 
nerve  cells  scattered  among  them,  the  nuclei  pontis,  and  pass  to  the  same 
or  opposite  side  of  the  cerebellum;  others  descend  from  the  cerebellum  to 
the  same  or  opposite  side  of  the  pons.     The  fibers  of  the  lemniscus  and 


Fig.  396. — Section  of  the  Medulla.  (After  Dejerine.) 
C.  r.,  Corpus  restiforme  ;  f.  c.  o.,cerebello-olivary  fibers  ;  lem., 
lemniscus;  n.  am.,  nucleus  ambiguus;  n.h.,  nucleus  hy- 
poglossi ;  ol.,  olive;  py.,  pyramid  ;  t.s.,  tractus  solitari- 
us  ;  t.  s.  n.  t.,  tractus  spinalis  nervi  trigemini ;  v.,  fourth 
ventricle. 


342 


HISTOLOGY. 


pyramidal  or  cerebrospinal  tract  traverse  the  pons,  together  with  a  bundle 
which  ascends  beyond  the  trigeminal  fibers  and  then  turns  back  to  enter 
the  cerebellum  through  the  brachium  conjunctivum.  (This  group  of 
fibers  ascending  to  the  cerebellum  is  found  in  the  superficial  ventro-lateral 
fasciculus  of  the  cord,  and  is  known  as  Gowers'  bundle.)  The  brachium 
conjunctivum  contains  fibers  from  the  cerebellum,  which  decussate  in  the 
mid-brain;  some  of  them  terminate  in  nuclei  which  send  branches  through 

the  pons  and  down  the  lat- 
eral bundles  of  the  cord; 
others  pass  into  the  cord 
directly 


■~"  Gray  stiattm. 

.  Ganglionic 

stratum. 

—  Granular  Ptratur 

Cortex. 


— "  Medulla. 

\ 
\ 


Cerebellum. 
The  medullated  nerve 
fibers  of  the  brachia  and 
restiform  body  form  an  ar- 
borizing medulla  which  ex- 
tends into  the  small  sub- 
divisions of  the  cerebellum 
as  shown  in  Fig.  397.  This 
medulla  of  white  substance 
is  surrounded  by  a  cortex 
consisting  of  an  inner  gran- 
ular stratum,  a  middle 
ganglionic  stratum  (present- 
ing in  section  a  single  row 
of  large  cell  bodies),  and  an 
outer  gray  stratum. 

The  inner  granular 
stratum,  which  is  rust-col- 
ored, consists  of  many  layers  of  small  cells  which  by  ordinary  methods 
show  relatively  large  nuclei  and  very  httle  protoplasm.  With  the  Golgi 
method  it  appears  that  besides  neurogha  cells,  two  sorts  of  nerve  cells  are 
present,  the  small  and  large  gramile  cells ;  the  former  (Fig.  398)  are 
multipolar  ganghon  cells  with  short  dendrites  having  claw-hke  terminations, 
and  a  slender  non-medull.ated  neuraxon  which  ascends  perpendicularly  to 
the  outermost  layer  and  there  divides  in  T-form  into  two  branches.  The 
branches  run  lengthwise  of  the  transverse  folds  or  convolutions  of  the 
cerebellum  so  that  they  are  cut  across  in  sagittal  sections  (Fig.  398);  they 
are  parallel  with  the  surface  and  have  free  unbranched  endings  (Fig.  399). 
The  small  granule  cells  form  the  bulk  of  the  granular  stratum.     The  less 


Fig.  397.— From  a  Sagittal  Skction  of  the  Cerebel- 
lum OF  AN  Adult  Man.    X  12. 


CEREBELLUM. 


343 


frequent  large  granule  cells  (Fig.  398)  are  more  than  twice  the  size  of  the 
small  ones;  their  branched  dendrites  reach  even  into  the  gray  stratum, 
and  their  neuraxon,  going  in  the  opposite  direction,  is  soon  resolved  into 
very  numerous  branches  which  penetrate  the  entire  granular  stratum. 


Collaterals  of  a 
cortical  cell 


Neuraxon  of  a 
large   cell  of  the 
granular  stratum. 
^  Fibers  to  the 
cortex. 

Small  cells  of  the  gran- 
ular stratum. 


P         ,    ^     ,  ^"'-  398.-D1.AGRAM  OF  A  Sagittal  Section  of  the  Cerebellum 

Except  the  large  granule  cel..^h....^,^,,,,^j,^ 

The  granular  layer  also  contains  a  thick  network  of  medullated  fibers 
which  proceed  chiefly  from  the  white  substance.  A  part  of  these  fibers 
end  in  the  "eosine  bodies"  of  the  granular  stratum,  which  are  heaps  of 


344 


HISTOLOGY. 


Fig.  399. — Diagram  of  a  Section  of. the  Cerebellum 
Lengthwise  of  the  Convolutions.  Golgi's 
Method.    (Kolliker.) 

gr.,  Cells  of  the  granular  stratum  ;  n.,  their  neuraxons  in 
the  granular  layer  and  n'.,  in  the  gray  stratum  ;  p.,  p'., 
Purkinje's  cells.    (From  Bailey's  "  Histology.  ") 


stainable  particles  found  between  the  small  cells  (Fig.  400).     Another 

part  of  the  fibers  forms  bundles,  parallel  with  the  surface,  running  between 

the  granular  and  ganghonic 
strata  in  the  sagittal  direc- 
tion ;  they  send  branches  into 
the  gray  layer.  A  small  por- 
tion of  the  granular  stratum 
is  formed  by  the  meduUated 
neuraxons  of  the  cells  in  the 
ganghon  layer. 

The  middle  ganglionic 
stratum  consists  entirely  of  a 
single  layer  of  very  large 
multipolar  ganghon  cells 
called  Purkinje's  cells.  Their 
oval  or  pear-shaped  bodies 
send  two  large  dendrites  into 
the  gray  stratum,  where  they 
form  an  extraordinar}^  arbor- 
ization (Fig.  398).  Their  many 

branches  do  not  extend  in  all  directions  but  are  confined  to  the  sagittal  plane, 

that  is,  to  a  plane  at  right  angles  with  the  long  axes  of  the  convolutions. 

When  the  convolutions  are  cut  lengthwise,  Purkinje's  cells  appear  as  in  Fig. 

399.     From  the    deep    surface    of   the 

cell  bodies    the   neuraxons    arise,    and 

as  medullated  fibers  they  pass  through 

the   granular    stratum     to    the    white 

substance.     Within  the   granular  layer 

they   produce    collateral    fibers    which 

branch  and  in  part  return  to  Purkinje's 

cell  bodies. 

The  outer  gray  stratum,    of  gray 

color,  contains  two  sorts  of  nerve  cells, 

the  large  and  the  small  cortical   cells. 

The  large  cortical  or   basket  cells    are 

multipolar  ganghon  cells  the  dendrites 

of  which  chiefly  pass  toward  the  sur- 
face.     Their    long  neuraxons,   thin  at 

first  but  later  becoming  thicker,    run    parallel  with    the   surface   in    the 

sagittal  plane.     They  send  occasional  collaterals  toward  the  surface,  and 

at   intervals  produce  finebranches  which  descend  and  terminate  in  baskets 


Eosine  bodies. 
1\ 


Nuclei  of  small  cells  of 
the  granular  stratum. 


\ 


f^'^ 
I^^" 


^^    * 


.9«^«® 


®^X  '  .«V*  «t 


^^®"    f 


3 


*^-f.*«^^.®^*®<§®^. 


a®-*'  7  -  ( 


(S>xiSS  . 


Fig.  400. —  From  a  Thin  Section  of  the 
Cerebellum  of  an  Adult.    X  400. 


CEREBELLUM.  345 

around  the  cell  bodies  of  Purkinje's  cells  (Fig.  398).  Often  the  basket 
surrounds  also  the  beginning  of  their  neuraxons. 

The  small  cortical  cells,  distinguished  from  the  basket  cells  suice 
their  neuraxons  are  not  in  relation  with  Purkinje's  cells,  may  be  divided 
into  two  types  connected  by  intermediate  forms.  The  cell  bodies  of  the 
first  type  are  nearly  or  quite  as  large  as  those  of  the  basket  cells.  Its 
two  to  five  dendrites  He  in  the  sagittal  plane  Hke  those  of  Purkinje's  cells; 
its  slender  neuraxon,  i  mm.  long  or  more,  sometimes  forms  loops,  and 
is  characterized  by  abundant  branches  in  its  proximal  part.  The  terminal 
branches  are  few.  The  second  type  is  in  general  somewhat  smaller;  the 
shorter  neuraxons  of  its  cells  branch  in  their  immediate  vicinity.  The 
elements  of  the  first  t}qDe  form  the  bulk  of  the  relatively  numerous  small 
cortical  cells,  and  are  found  throughout  the  gray  stratum,  though  they  are 
more  abundant  in  its  superficial  part.  The  second  type  appears  every- 
where in  the  gray  stratum. 

The  medullated  ner\-e  fibers  found  in  the  gray  layer  are  prolongations 
from  the  granular  stratum.  In  part  they  proceed  toward  the  surface 
where,  after  losing  their  myehn,  they  end  in  branches  among  the  dendrites 
of  Purkinje's  cells;  in  part  they  run  between  the  bodies  of  Purkinje's 
cells  lengthwise  of  the  convolutions. 

The  neuroglia  of  the  cerebellum  consists  of  short-rayed  stellate  cells 
found  in  all  the  layers;  of  long-rayed  cells  found  in  the  white  substance; 
and  of  peculiar  cells  with  small  bodies  found  at  the  outer  boundar}^  of  the 
granular  layer.  These  send  only  a  few  short  processes  inward,  but  many 
long  processes  straight  out  to  the  free  surface,  where  they  end  in  triangu- 
lar expansions.  In  this  way  a  thick  peripheral  neurogha  layer  is  produced. 

As  long  as  the  cerebellar  cortex  is  not  fuUy  developed  it,  presents  a 
series  of  peculiarities  which  are  lacking  in  the  adult.  Thus  in  embryos 
and  yoimg  animals  the  partly  developed  gray  stratum  is  covered  by  a 
superficial  granular  layer,  the  cells  of  which  later  become  ner\-e  and  neu- 
roglia cells  of  the  cortex. 

Hemispheres. 

The  ascending  fibers  of  the  lemniscus  and  the  descending  cerebro- 
spinal or  pyramidal  tracts,  continue  from  the  meduUa  through  the  pons  and 
peduncles  of  the  cerebrum  into  the  hemispheres.  They  enter  them  on 
each  side  between  the  thalamus  and  the  lentiform  nucleus  (a  subdivision 
of  the  corpus  striatum)  as  seen  in  Fig.  401.  The  fibers  of  the  ascending 
tract  have  received  accessions  from  the  cerebral  nerves,  the  thalamus, 
corpora  quadrigemina,  and  other  special  nuclei  near  which  they  pass. 
Many  of  the  fibers  which  arise  in  the  gracile  and  cuneate  nuclei  terminate 


346 


HISTOLOGY. 


before  reaching  the  hemispheres  and  their  course  is  prolonged  by  a  new 
set  of  nerve  cells. 

The  central  portion  of  the  hemispheres  is  a  mass  of  white  substance. 
The  peripheral  zone  of  gray  in  which  these  fibers  arise  or  terminate  is  called , 
the  cortex;  it  is  divided  into  four  ill-defined  layers,  an  outer  molecular 
or  neurogha  layer,  a  layer  of  small  pyramidal  cells,  a  layer  of  large  pyram- 
idal cells,  and  next  the  white  substance,  a  layer  of  polymorphous  cells. 
From  the  pyramidal  cells  the  fibers  of  the  descending  tract  arise.  The 
layers  are  shown  in  Figs.  403  and  404. 

The  molecular  layer  which  in  ordinary  sections  appears  finely  punctate 
or  reticular  contains,  besides  many  neurogha  cells,  a  network  of  medullated 

fibers  parallel  with  the  surface,  the  tangen- 
tial fibers.  The  Golgi  method  shows  that 
these  fibers  are  partly  neuroglia  and  partly 
the  dendrites  of  pyramidal  cells.  The 
''cells  of  Retzius"  found  in  this  layer  have 
irregular  bodies,  and  processes  some  of 
which  are  parallel  with  the  surface;  their 
branches,  together  with  other  processes  from 
the  cell  body,  descend  into  the  pyramidal 
layer  (Fig.  402).  These  cells  are  probably 
neuroglia. 

The  layer  of  small  pyramidal  cells  is 
characterized  by  large  ganglion  cells  with 
pyramidal  bodies  measuring  10-12/^,  Since 
they  taper  into  a  dendritic  process  their 
length  cannot  be  definitely  determined. 
The  chief  dendrite,  after  producing  small 
lateral  branches,  enters  the  molecular  layer 
its  terminal  branches  often  show  small, 
irregular  projections.  Lesser  dendrites  proceed  from  the  sides  and  basal 
surface  of  the  pyramidal  cell  body.  The  neuraxon  always  arises  from 
the  basal  surface,  and  after  producing  branched  collaterals  it  generally 
enters  the  white  substance  where  it  may  divide  in  two  (Fig.  402,  3). 
Sometimes  the  neuraxon  turns  toward  the  molecular  layer,  joining  the 
tangential  fibers;  infrequently  an  inverted  pyramidal  cell  is  found.  The 
neuraxons  and  collaterals  are  medullated. 

The  layer  of  large  pyramidal  cells  contains  those  with  bodies  20-30  jf. 
long  (the  "giant  pyramidal  cells"  of  the  anterior  central  convolution 
measure  even  80  //.).  The  very  large  neuraxon  always  goes  to  the  white 
substance,  after  sending  out  several  collaterals  in  the  gray. 


Fig.  401.  —  Transverse  Section  of 
THE  Brain.    About  J^  natural  size. 

The  gray  substance  is  stippled ;  the 
white  is  blank,  a.  t.,  Ascending 
tract,  including  the  fillet;  c.  C, 
corpus  callosum  ;  d.  t.,  descending 
tract,  entering  the  hemisphere  from 
the  cerebral  peduncle  ;  n.  I.,  nucleus 
lentiforniis  ;  th.,  thalamus;  v., third 
ventricle. 


where   it    arborizes    freely; 


HEMISPHERES. 


347 


The  layer  of    polymorphous  cells  includes    oval  or  polygonal  cells 
which  lack  a  chief  dendrite  directed  toward  the  surface;    their  slender 


Cell  of  Retzius. 
^.   Short-rayed  neuroglia  cell. 
Blood  vessel. 


Neuraxon  ot 
a  polymor- 
phous nerve 
cell. 


Long-rayed- 
neuroglia  cell. 


Fig.  402. — Diagram  of  the  Cerebral  Cortex.    The  cells  on  the  right  are  drawn  from  Golgi  prep- 
arations of  an  adult  man.     X   120.     The  left  portion  of  the  diagram  is  X  60. 

neuraxons  produce  collaterals,  and  enter  the  white  substance  where  they 
may   divide   into   two   branches  in   T   form.     Polymorphous    cells    with 


348 


HISTOLOGY. 


Pia  mater. 


Tangential    --<=^;'  rZ'S^ 
network.        '"  ^ 


,;,>jS>f>tW352,  V^?^ 


Supra- 
radial 
network. 


Inter-radial    ■'"r^?^,; 
network.  ^ 


Radial 
bundles 


Medulla  or 

white  sub 

stance. 


;;•   J 


-iiVL- 


i  * 


r  ^\ 


11. 


iMifey,:!'; 


Molecular 
layer. 


Layer  of  small 

pyramidal 

cells. 


Blood  vessel. 


Layer  of  large 

pyramidal 

cells. 


•:-li-^-: 


i'.   ■   ;• 


'l-y 


1 
.11 


i;..,    I 


Layer  of  poly- 
morphous 
nerve  cells. 


't'  'O  *     •:  -  ,•   •'•  /'.'    ■■•■ 


'•¥/ 


J 


Fig.  404. 

Figs.  403  and  404  are  from  vertical  sections  of  the  cortex 
(central  convolution)  of  an  adult.  Fig.  403  is  a  VVeigert 
Ijrejjaration ;  Fig.  404  is  from  a  section  stained  with 
haematoxyline  and  eosine.     X  45- 


Fig.  403. 


HEMISPHERES.  349 

branched  neuraxons  limited  to  the  vicinity  of  the  cell  body  are  found  in 
this  layer  and  in  the  pyramidal  layers  also.  The  neuraxon  may  branch 
in  the  molecular  layer  (Fig.  402,  6). 

Many  medullated  fibers  are  found  in  the  deeper  pyramidal  and 
polymorphous  layers.  They  are  grouped  in  tapering  radial  bundles 
which  are  resolved  into  separate  fibers  toward  the  layer  of  small  pyramidal 
cells  (Fig.  403).  The  bundles  include  the  descending  medullated  neu- 
raxons of  the  pyramidal  cells,  and  the  ascending  medullated  fibers  from  the 
white  substance,  which  end  after  branching  repeatedly  in  the  supra-radial 
and  tangential  networks.  The  medullated  collaterals  of  the  pyramidal 
cells  run  at  right  angles  with  the  radial  bundles;  they  form  an  inter-radial 
network,  or  a  band  of  fibers  which  near  the  calcarine  fissure  is  macroscopic. 
A  similar  supra- radial  band  may  be  detected  elsewhere  in  thick  sections. 

In  the  gyrus  hippocampi  and  its  hook  (uncus)  the  tangential  fibers 
are  so  abundant  as  to  form  a  considerable  layer,  the  substantia  reticularis 
alba.  The  hippocampus  [Amnion's  horn],  olfactory  bulb,  and  some  other 
areas  of  the  cortex,  differ  in  details  from  the  central  region  which  has  been 
described ;  these  peculiarities  are  considered  in  the  larger  special  works  on 
the  nervous  system. 

The  neuroglia  of  the  hemispheres,  hke  that  of  the  cord,  is  at  first  a 
syncytium  with  strands  extending  from  the  ventricle  to  the  periphery. 
Later,  the  syncytium  is  divisible  into  short-rayed  neuroglia  cells  found 
chiefly  in  the  gray  substance,  long-rayed  cefls  found  chiefly  in  the  white, 
and  ependymal  cells  lining  the  ventricles.  The  ependymal  layer  is  con- 
tinuous through  the  aqueduct  with  that  of  the  fourth  ventricle  and  central 
canal.  In  early  stages  its  cells  have  cilia-like  processes  which  are  in  part 
retained  in  the  adult.  The  short-rayed  cells,  which  are  characterized  by 
knotted,  branching  processes,  are  often  in  close  relation  with  the  blood 
vessels;  they  may  serve  to  transfer  the  nutritive  and  myehn-forming 
material  from  the  vessels  to  the  nerve  fibers.  The  periphery  of  the 
cerebral  cortex  is  particularly  rich  in  neurogha  fibers. 

Hypophysis. 

The  development  of  the  two  lobes  of  the  hypophysis  [pituitary 
body],  the  anterior  from  the  oral  ectoderm  and  the  posterior  from  the 
telencephalon,  has  already  been  described  (Fig.  185,  p.  165).  The 
smaller  posterior  lobe,  which  is  at  the  tip  of  the  inf undibulum,  contains  fine 
branching  nerve  fibers  which  form  a  dehcate  network,  together  with 
cefls  closely  resembhng  bipolar  and  multipolar  ganglion  cefls,  and  many 
blood  vessels.  The  nature  of  the  cells  is,  however,  uncertain.  The 
larger  anterior  lobe  consists  of  loose  connective  tissue  with  many  blood 


35° 


HISTOLOGY. 


vessels  and  nerves,  and  of  solid  branched  epithelial  cords  varying  in 
caliber  and  frequently  anastomosing.  Near  its  border  toward  the  posterior 
lobe  a  few  of  the  columns  are  hollow,  and  sometimes  they  contain  masses 
similar  to  the  colloid  of  the  thyreoid  gland.  This  does  not  come  from  the 
granules  which  occur  in  varying  quantity  in  all  the  epithelial  cells,  giving 
them  sometimes  a  lighter  and  sometimes  a  darker  appearance.  The 
granules  in  some  cells  are  eosinophihc ;  most  of  them  are  not,  and  a  portion 
may  be  fat.  Ciliated  epithelial  cells  have  been  recorded.  (The  part  of 
the  anterior  lobe  which  is  near  the  posterior  is  sometimes  called  "medullary 
substance";  in  children  it  may  be  represented  by  a  cleft-hke  cavity  con- 
taining colloid).  From  the  relation  of  the  hypophysis  to  certain  diseases, 
it  is  quite  certain  that  it  produces  an  important  internal  secretion. 


Epithelial  cord. 


Portion  of  the 
anterior  lobe. 


'm.^ 


Portion  of  the 
posterior  lobe. 


J  Blood  vessel  con- 

^^*  taining  blood 

corpuscles. 


•^1~  "Colloid" 
-*'        substance. 


Multipolar  cell. 


Connective  tissue 
fibers. 


Fig.  405.— Portion  of  a  Horizontal  Section  of  a  Human  Hypophysis,  showing  the  boundary 
line  between  the  anterior  and  the  posterior  lobes.  Two  gland  follicles  on  the  left  each  contain  a 
dark  epithelial  cell.     X  220. 


Pineal  Body. 

The  pineal  body  [epiphysis]  is  a  median  dorsal  outpocketing  of  the 
diencephalon,  which  has  preserved  its  original  epithelial  character.  It 
consists  of  a  layer  of  neuroglia  cells  thrown  into  folds  and  is  covered  by 
a  connective  tissue  capsule  sending  prolongations  between  the  folds.  In 
the  pineal  body  there  is  found  generally  "brain  sand,"  acervulus  cerebri, 
which  consists  of  round  or  mulberry-like  concretions  ^/i  to  i  mm.  in  diameter. 
In  specimens  preserved  in  glycerin  or  balsam  they  show  distinct  concentric 
layers.  They  consist  of  an  organic  matrix  containing  calcium  carbonate 
and  magnesium  phosphate,  and  are  sometimes  surrounded  by  a  thick 
connective  tissue  capsule. 

Not  infrequently,  especially  in  old  age,  there  are  found  in  the  brain  sub- 
stance round  or  elongated  bodies  distinctly  stratified,  which  are  colored 


MENINGES.  351 

violet  by  tincture  of  iodine  and  sulphuric  acid,  and  therefore  are  related  to 
amyloid.  These  corpuscula  amylacea  are  found  almost  always  in  the 
walls  of  the  ventricles  of  the  brain,  and  also  in  many  other  places  both  in 
the  gray  and  white  substance  and  in  the  optic  nerve.  They  have  a  homo- 
geneous capsule  with  occasional  processes,  composed  of  neuroglia  cells 
transformed  by  amyloid  infiltration. 

Meninges. 

The  meninges  are  connective  tissue  membranes  investing  the  central 
nervous  system.  They  are  usually  divided  into  three  layers,  the  dura 
mater,  arachnoid,  and  pia  mater. 

The  dura  mater  spinalis,  or  dura  mater  of  the  cord,  consists  of  com- 
pact fibrous  connective  tissue  with  many  elastic  fibers,  flat  connective 
tissue  cells  and  plasma  cells.  Its  inner  surface  is  covered  by  a  layer  of 
flat  cells  forming  a  mesenchymal  epithelium.  It  has  few  nerves  and  blood 
vessels.  The  dura  mater  cerebral  is  or  dura  mater  of  the 
brain,  includes  the  periosteum  of  the  inner  surface  of  the  ^\ 

cranium  and  consists  of  two  lamellae.     The  inner  is  Hke  ^fv 

the  dura  mater  of  the  cord  but  contains  more  elastic  ^'    _- 

fibers;  the  outer  corresponds  with  the  periosteum  of  f 

the  vertebral  canal  and  consists  of  the  same  elements         %:v't  ;,r^--^ 
as  the  inner  layer,  but  its  fibers  run  in  a  different  direc-  H^y^  t 

tion.     It  contains  many  blood  vessels,  some  of  which  ^^1^ 

extend  into  the  cranial  bones.      The  very   large   thin-      fig.  4o6.-acervulus 

,  .  r       1  1  11        1         •  rrn  1  FROM    THE    PiNEAL 

walled  vems  of  the  dura  are  called  sinuses.     1  ne  dura  body  of  a  woman 

Seventy    Years 

has  many  nerves,  some  endmg  freely  and  others  supply-  old.   x  50. 

ing  the  vessels. 

The  arachnoid  of  the  cord  and  brain  is  but  loosely  connected  with  the 
dura,  being  generally  limited  externally  by  a  mesenchymal  epithelium. 
Between  the  arachnoid  and  the  dura  there  is  a  capillary  cleft  containing 
a  very  small  amount  of  fluid.  This  subdural  space  in  the  rabbit  and  dog 
is  in  communication  with  the  deep  cervical  lymphatic  vessels  and  glands, 
with  the  lymphatic  spaces  around  the  peripheral  nerves,  with  the  lymphatic 
vessels  of  the  nasal  mucosa,  with  the  tissue  spaces  in  the  dura,  and  with  those 
around  the  arachnoid  granulatious. 

The  arachnoid  granulations  [Pacchionian  bodies]  are  elevations  or 
outpocketings  of  the  arachnoid  in  definite  places,  especially  along  the  sides 
of  the  superior  sagittal  sinus.  Covered  by  a  thin  portion  of  the  dura  and 
by  the  endothelium  of  the  vessel,  they  project  into  the  cavity  of  the  sinus. 

The  subarachnoid  space  between  the  arachnoid  and  the  pia  mater,  is 
traversed  by  strands  and  layers  of  tissue  and  bounded  by  mesenchymal 


352  HISTOLOGY. 

epithelium.  It  connects  with  the  lymph  spaces  of  the  peripheral  nerves, 
with  the  lymph  vessels  of  the  nasal  mucosa,  and  with  the  ventricles  of  the 
brain  through  apertures  in  the  roof  of  the  fourth  ventricle.  It  contains 
an  abundant  fluid  called  the  liquor  cerebrospinalis.  (The  direct  con- 
nection of  the  subdural  and  subarachnoid  spaces  with  both  lymphatic 
vessels  and  tissue  spaces,  is  not  in  accord  with  recent  embryological  studies 
and  requires  further  investigation.) 

The  pia  mater  of  the  cord  and  brain  is  a  delicate  vascular  connective 
tissue  which  extends  into  their  substance  along  with  its  blood  vessels. 
Its  nerves  may  remain  outside.  Pericellular  lymphatic  spaces  around 
the  nerve  cells,  and  the  epicerebral  space  between  the  pia  and  the  brain,  do 
not  communicate  directly  with  the  lymphatic  vessels.  The  blood  vessels 
form  narrow-meshed  capillaries  in  the  gray  substance  and  coarser  ones 
in  the  white.     Capillaries  in  the  cerebral  cortex  empty  into  veins  which 

Blood  vessels.  x  Epithelium. 


^^i 


Fig.  407.— Portion  of  the  Ple.xus  Chorioideus  of  a.n   Adult   Man.     X  80. 
X,  Blood  vessel  in  optical  section.     The  large  dots  in  the  epithelium  are  not  nuclei,  but  pigment  and 

fat  granules. 

arise  in  the  white  substance  beneath,  and  from  there  pass  through  the 
cortex  to  the  pia;  the  blood  in  the  capillaries  therefore  passes  through  the 
entire  cortex  before  emptying  into  the  veins.  The  blood  vessels  generally 
have  a  second  so-called  "adventitial  sheath"  consisting  of  a  mesenchymal 
epithelium.  Within  the  sheath  is  an  "adventitial  lymph  space"  con- 
necting with  the  subarachnoid  space;  outside  of  it  is  a  perivascular 
tissue  space. 

Chorioid  plexuses.  In  certain  places  where  the  wall  of  the  medullary 
tube  is  very  thin,  as  in  the  roof  of  the  fourth  ventricle,  it  becomes  invagi- 
nated  into  the  central  cavity  by  the  vascular  pia,  thus  forming  a  chorioid 
plexus.  The  epithehal  cells  of  the  brain  covering  the  plexus,  contain 
pigment  granules  and  sometimes  fat  droplets.  The  chorioid  plexuses 
extending  into  the  third,  fourth,  and  both  lateral  ventricles,  are  essentially 
similar  in  structure.  A  part  of  the  network  of  blood  vessels  within  them 
is  shown  in  Fig.  407. 


DE\"ELOPiIEXT    OF    THE    EYE.  353 


EYE. 


DE\'ELOPiIEXT   AXD    GeXEILIL   AXATOilY. 

The  eyes  first  appear  as  a  pair  of  optic  vesicles,  which  are  lateral  out- 
pocketings  of  the  fore-brain.  They  are  shown  in  the  model,  Fig.  390,  A 
(p.  32 5  j  and  in  section  in  Fig.  409,  A.  The  vesicles  are  connected  with  the 
brain  by  the  optic  stalks,  which  become  relatively  slender  as  the  vesicles 
enlarge.  The  epidermal  ectoderm  immediately  overlying  the  vesicles, 
thickens  and  becomes  invaginated  (Fig.  409,  B  and  C).  The  invaginated 
portion  then  becomes  detached  in  the  form  of  a  vesicle,  the  inner  wall  of 
which  is  distinctly  thicker  than  the  outer;  this  "'lentic  vesicle"  becomes 
the  lens  of  the  eye.  Meanwhile,  as  seen  in  B  and  C,  that  layer  of  the  optic 
vesicle  which  is  toward  the  surface  is  pressed  in,  transforming  the  vesicle 
into  the  optic  cup.  At  first  the  cup  is  not  complete,  being  deficient  on  its 
lower  side  (Fig.  408) .  The  arteria  centralis  retinae  is 
seen  passing  through  the  indentation,  which  begins  on 
the  lower  surface  of  the  stalk  and  extends  to  the  free 
margin  of  the  cup ;  the  cleft  is  sometimes  called  the 
"  chorioid  fissure."  Distal  to  the  point  of  entrance  of 
the  artery  into  the  optic  cup  the  edges  of  the  fissure 
fuse;  the  artery  then  appears  to  perforate  the  base 
of  the  cup,  and  it  retains  this  relation  in  the  adult.       fig.4o8.-opticCupand 

i  '  Stalk  of  a   HuiiAX 

The  arter}^  is  sho^^m  in  section  in  Fig.  409,  D.  uftCT  KoUmann  )*™' 

The  two  layers  of  the  optic  cup,  the  inner  of 
which  is  toward  the  lens,  are  normaUy  in  contact  with  one  another,  al- 
though in  sections  they  are  often  more  or  less  separated.  They  constitute 
the  retina,  which  includes  a  thin  outer  pigmented  layer,  and  a  thick  inner 
visual  layer  composed  of  several  strata  of  nen-e  cells  and  fibers.  The 
stimulus  of  hght  is  received  by  tapering  projections  extending  from  the 
outer  surface  of  the  \^sual  layer  toward  the  pigmented  layer;  to  reach 
them  the  rays  of  hght  must  traverse  the  strata  of  the  ^dsual  layer.  In 
explanation  of  the  fact  that  the  sensory  processes  are  turned  away  from 
the  hght  it  is  stated  that  the  outer  surface  of  the  skin  ordinarily  receives 
stimuH,  and  that  through  the  infolding  which  makes  the  medullary 
tube  and  the  outpocketing  which  makes  the  optic  vesicle,  the  sensory  sur- 
face of  the  retina  is  seen  to  be  continuous  with  the  outer  surface  of  the 
skin.  Smce  in  mammals  the  optic  vesicles  begin  to  form  before  the  rela- 
ted portion  of  the  medullar}^  groove  has  closed,  they  appear  as  depressions 
in  a  thickened  epidermal  ectoderm. 

Nerve  fibers  grow  from_the  inner  surface  of  the  visual  layer  toward 
23 


354  HISTOLOGY. 

the  central  artery  and  vein  of  the  retina,  around  which  they  pass  out  of  the 
optic  cup  (Fig.  409,  D).  They  grow  beneath  and  among  the  cells  of  the 
optic  stalk  to  the  brain,  which  they  enter.  These  fibers  which  constitute 
the  optic  nerve,  cause  the  obUteration  of  the  optic  stalk.  It  is  shown  in  the 
figure  that  the  optic  nerve  at  its  origin  interrupts  the  retinal  layers,  pro- 
ducing a  "blind  spot."  The  part  of  the  nerve  which  forms  the  blind 
spot,  with  the  vessels  in  the  center,  is  called  the  papilla  0}  the  optic 
nerve. 

The  lens  (Fig.  409,  D)  loses  its  central  cavity  by  the  elongation  of  the 
cells  in  its  posterior  layer.  These  become  the  -fibers  of  the  lens.  The 
anterior  layer  remains  throughout  life  as  a  simple  epithelium,  called  the 
epithelium  of  the  lens  (Fig.  410).  The  lens  becomes  covered  by  an 
elastic  capsiila  lentis  and  in  embryonic  life  it  possesses  a  vascular  capsule 
Fig.  409,  E)  containing  branches  of  the  central  artery.  The  vascular 
layer  covering  the  anterior  surface  of  the  lens  is  designated  the  pupillary 
membrane,  and  it  disappears  shortly  before  birth.  Its  occasional  persistence 
interferes  with  vision. 

Between  the  lens  and  the  retina  there  is  a  pecuHar  tissue,  mucoid  in 
appearance  and  resembling  mcsenchyma  in  form.  Since  processes  from 
the  retina  and  from  the  lens  have  been  found  extending  into  it,  it  is  con- 
sidered to  be  essentially  ectodermal.  Its  blood  vessels  become  obliterated 
and  it  forms  the  vitreous  body  of  the  adult,  consisting  of  a  stroma  and  a 
humor.  Extending  through  it,  from  the  papilla  of  the  optic  nerve  toward 
the  lens,  is  the  hyaloid  canal,  which  in  the  embryo  lodged  the  hyaloid  artery 
(a  prolongation  of  the  central  artery).  Sometimes  this  artery  is  repre- 
sented in  the  adult  by  a  strand  of  tissue.  The  vitreous  body  is  surrounded 
by  a  fibrous  layer  called  the  hyaloid  membrane. 

A  cavity  forms  in  the  tissue  in  front  of  the  lens  and  becomes  filled  with 
a  watery  tissue  fluid  (aqueous  humor).  It  is  bounded  by  a  mesenchymal 
epithelium.  The  portion  of  the  cavity  which  is  anterior  to  the  retinal  cup 
and  lens  is  called  the  anterior  chamber  of  the  eye;  the  smaller  part  within  the 
retinal  cup  but  in  front  of  the  lens  and  the  fibrous  covering  of  the  vitreous 
body,  is  the  posterior  chamber  (Fig.  309,  E,  c.p). 

The  retinal  cup  is  surrounded  by  two  layers  of  mesenchymal  origin. 
The  inner  tunica  vasculosa  corresponds  with  the  pia  mater  and  forms  the 
chorioid  coat  of  the  eye ;  the  outer  tunica  fibrosa  corresponds  with  the  dura 
and  forms  the  sclera,  into  which  the  muscles  of  the  eye  are  inserted.  The 
portion  of  the  retinal  cup  which  forms  a  curtain,  circular  in  front  view, 
between  the  anterior  and  posterior  chambers,  is  called  the  iris.  It  consists 
of  the  tunica  vasculosa  together  with  a  thin  pigmented  prolongation  of  the 
retina  over  its  posterior  surface  (Fig.  410).     This  pars  iridica  retinae  is 


DEVELOPilEXT    OF    THE    EYE. 


355 


rudimentan'  and  without  visual  function.  The  iris  is  covered  by  the  mesen- 
chymal epithehum  of  the  chambers.  At  the  attached  border  of  the  iris  the 
vascular  coat  contains  important  muscle  fibers  and  is  there  thickened  to 
form  the  ciliary  body.  This  is  also  covered  by  a  rudimentar}^  pigmented 
layer  on  its  inner  surface,  the  pars  ciliaris  retinae.  At  the  ora  serrata  (Fig. 
425)  an  abrupt  thickening  of  the  \dsual  layer  of  the  retina  marks  the  boun- 


/-el 


Fig.  409.  —  Sections  of  Rabbit  Embryos  to  show  the  Development  of  the  Eye.  A,  gj^  days, 
3.0  mm.;  B,  10%  days,  5.4  mm.;  C,  11  days,  5.0  mm.;  D,  14  days,  18  hours,  12.0  (?)  mm.;  E,  20  days,  29  mm. 

a.  C.  r.,Arteria  centralis  retinae:  c,  cornea;  c.  a.,  anterior  chamber :  conj..  conjunctiva  ;  c.  p.,  posterior 
chamber ;  c.  v.,  corpus  vitreum  ;  e.  I.,  evelid  :  f.  b..  fore-brain  :  I.,  lens  ;  I.  e.,  lens  epithelium  ;  I.  f.,  lens 
fibers;  0.  C,  optic  cup ;  0.  n.,  optic  nerve;  0.  v.,  optic  vesicle:  r.  p.,  pigmented  layer  ot  the  retina; 
r.  v.,  visual  layer  of  the  retina. 

dary  between  its  ciliary  and  optic  portions.  The  pars  optica  retinae  extends 
from  the  ora  to  the  optic  ner\-e,  covered  successively  by  the  chorioid  and 
sclera. 

The  cornea  is  the  tissue  ui  front  of  the  anterior  chamber,  consisthig  of 
a  non- vascular  mesenchymal  tissue  bounded  posteriorly  by  mesenchymal 


356 


HISTOLOGY. 


epithelium  and  anteriorly  by  the  epidermal  ectoderm.  The  cornea  is 
extremely  transparent.  The  epidermal  ectoderm  extends  from  the  cornea 
over  two  folds  which  form  the  eyelids.  They  have  met  in  Fig.  409,  D, 
and  fused  temporarily.  Externally  the  lids  are  covered  by  skin,  internally 
by  the  conjunctiva  palpebrarum,  or  conjunctiva  of  the  lids.  The  latter  is 
continuous  with  the  conjunctiva  bulbi  which  forms  the  opaque,  vascular 


..   Epithelium  -i 

.  -  Anterior  basal  lamina  I 

,'  Substantia  propria  [►  of  the  cornea. 

.  Posterior  basal  lamina  | 

Mi'senchymal   epithelium  J 


Si)hincter  muscle  ") 

Stroma  I- of  the  iris. 

Pars  iridica  retinae) 

Anarle  of  the  iris.        Sinus  venosus  sclerae. 


Epithelium")        of  the 
Tunica  ,-  conjunctiva 

propria     j        bulbi. 


Circular    Meridional     Pars  ciliaris  retinae. 
Zonula.     Ciliary  process.      muscle  fibers. 
Corpus  ciliare. 


Capsule  1 
V   Epithelium  i  of  the  lens. 

Fibers  I 


Fig.  410. — Meridional  Section  of  a  Part  of  the  Eye.     X  15. 
The  radial  fibers  of  the  ciliary  muscle  cannot  be  distinguished  with  this  magnification. 


"white  of  the  eye."     It  surrounds  the  cornea,  the  epithelium  of  the  two 
structures  forming  an  uninterrupted  layer. 

The  parts  of  the  eye  to  be  examined  histologically  are  therefore  the 
retina,  the  optic  nerve,  the  lens,  and  the  vitreous  body,  all  of  which  are 
ectodermal;  then  the  tunica  vasculosa  including  the  chorioid,  ciliary 
body,  and  iris;  next  the  tunica  fibrosa,  including  the  sclera  and  cornea; 
and  finally  the  accessory  structures, — the  lids,  conjunctivae  and  glands. 


structure  of  the  eye.  357 

Retina. 

The  retina  extends  from  the  papilla  of  the  optic  nerve  to  the  pupillary 
border  of  the  iris,  and  is  divisible  into  three  parts;  the  pars  optica  retinae 
includes  all  vv^hich  is  actually  connected  with  the  optic  nerve  and  which 
therefore  is  sensitive  to  light.  It  covers  the  deeper  portion  of  the  optic 
cup,  ending  near  the  cihary  body  in  a  macroscopic  sharp,  irregular  line 
bounding  the  ora  serrata.  The  pars  ciliaris  and  the  pars  iridica  retinae 
are  the  rudimentary  layers  covering  the  ciliary  body  and  iris  respectively. 

The  pars  optica  retinae  in  a  fresh  condition  is  a  transparent  layer 
colored  reddish  by  the  "visual  purple."  '  In  sections  it  presents  many  layers 
arranged  as  seen  in  Fig.  411,  the  cells  of  which  are  related  to  one  another 
as  in  the  diagram,  Fig.  412.  The  outer  layer  of  the  optic  cup  forms  the 
pigmented  epithelium  of  the  retina,  which  consists  of  a  simple  layer  of  six- 
sided  cells.  Toward  their  outer  surface  (that  next  the  chorioid,  where  the 
nucleus  lies)  they  are  poor  in  pigment,  whereas  in  their  inner  portion  they 
contain  numerous  rod-shaped  (1-5  u-  long)  brown  granules  of  the  pigment 
"fuscin."  In  albinos  the  pigment  is  lacking.  From  the  inner  surface  of 
the  pigmented  epithehum  numerous  processes  extend  between  the  rods  and 
cones. 

The  visual  cells,  which  are  found  along  the  outer  surface  of  the  inner 
retinal  layer,  are  of  two  sorts,  rod  cells  and  cone  cells.  In  both,  the  nucleus 
is  found  in  the  inner  half  of  the  cell,  and  the  outer  non-nucleated  half 
projects  through  a  membrane,  the  membrana  limitans  externa.  This 
causes  the  visual  cells  to  appear  divided  into  two  layers,  their  nucleated 
parts  beneath  the  limiting  membrane  constituting  the  outer  nuclear  layer 
(or  outer  granular  layer),  and  the  non-nucleated  parts  outside  of  the  mem- 
brane forming  the  layer  of  rods  and  cones. 

The  rods  are  four  times  as  numerous  as  the  cones.  They  are  regularly 
placed  so  that  three  or  four  rods  are  found  between  every  two  cones  (Fig. 
411).  The  rods  are  elongated  cylinders  (60  /j.  long  and  2  /j-  thick)  con- 
sisting of  a  homogeneous  outer  segment  in  which  the  visual  purple  is  found 
exclusively,  and  a  finely  granular  inner  segment.  In  the  outer  third  of 
the  inner  segment  there  is  said  to  be  an  ellipsoid,  vertically  striated  struc- 
ture (which  in  some  lower  vertebrates  is  very  distinct) .  The  portion  of  the 
rod  cells  below  the  limiting  membrane  is  a  slender  thread,  expanding  to 
surround  the  nucleus  which  is  characterized  by  from  one  to  three  trans- 
verse bands.  Beneath  the  nucleus  the  protoplasm  again  becomes  thread- 
like and  terminates  in  a  small  club-shaped  enlargement  without  processes 
(Fig.  412). 

The  cones  likewise  consist  of  an  outer  and  an  inner  segment.     The 


358 


HISTOLOGY. 


conical  outer  segments  are  shorter  than  those  of  the  rods.  The  inner 
segments  are  thick  and  somewhat  dilated  so  that  the  entire  cone  is  flask- 
shaped.  Moreover,  the  inner  segment  contains  a  vertically  striated  "fiber 
apparatus."  The  nuclei  of  the  cone  cells  are  situated  just  beneath  the 
limiting  membrane;  below  the  nuclei  the  protoplasm  forms  a  fiber  ending 
in  an  expanded  pyramidal  base. 

Beneath  the  outer  nuclear  layer  there  is  a  zone  of  fibers  called  the 
outer  reticular  layer  [outer  molecular  layer].  It  contains  but  few  nuclei. 
The  basal  fibers  of  the  visual  cells  are  sometimes  described  as  forming  its 
outer  part;  more  specifically  they  are  called  Henlc's  fiber  layer.     The 


Pigmented 
epithelium. 


Laver  of  rods  and 


Membrana  limitans 
externa. 

Outer  nuclear  j 
laver.  i 

Henle's  fiber  layer.  — ■>,  "■  , 
Inner  reticular      >,.< 
laver.  ->■  ' 

f  my 


^lfl»- 


Vessels  of  the 
choriocapil- 
laris. 

Lamina  basalis. 


ner 
ment. 


Base  of  a  cone  fiber. 


Inner  nuclear  j 
layer.  | 


Inner  reticular  J 
layer.  1 

I 

Ganglion  cell  layer. 
Nerve  fiber  layer. 

Membrana  limitans 
interna. 


Nucleus  of  a  radial 
fiber. 


Nucleus  of  an 
amakrine  cell. 


Pyramidal  base  of  a 
radial  fiber. 


Blood  vessels. 
Fig.  411.— Vertical  Section  of  a  Human  Retina.     X  36. 


remaining  portion  is  a  dense  network  of  the  branching  processes  from 
underlying  nerve  cells.  Occasionally  a  cell  body  is  displaced  outward 
from  the  deeper  layer  and  comes  within  the  reticular  layer.  One  of  such 
"subepithelial  ganglion  cells"  is  seen  in  Fig.  412,  x.  The  nervous  ele- 
ments are  supported  by  a  fibrillar  network  derived  from  non-nervous 
ectodermal  cells,  corresponding  with  neurogha.  Some  of  the  supporting 
cells  found  in  the  reticular  layer  are  concentrically  arranged  (Fig.  412,  00). 
The  inner  nuclear  layer,  which  underlies  the  outer  reticular  layer, 
contains  the  cell  bodies  of  both  nerve  and  sustentacular  cells.  The  nuclei 
of  the  latter  belong  chiefly  with  radial  fibers  [Miiller's  fibers];  these  extend 
from  the  inner  surface  of  the  retina  to  the  membrana  limitans  externa, 


RETINA. 


359 


which  they  form.  Slender  fibers  which  arise  from  the  outer  surface  of 
this  membrane  and  surround  the  bases  of  the  rods  and  cones  in  the  form 
of  baskets,  may  be  regarded  as  prolongations  of  the  radial  fibers.  The 
inner  ends  of  the  radial  fibers  form  pyramidal  expansions  which  unite  with 
one  another  to  make  a  membrana  limitans  interna, — the  innermost  layer 
of  the  retina.  Throughout  their  course  the  radial  fibers  give  off  lateral 
expansions  and  processes,  for  the  support  of  the  nervous  elements;  these 
are  especially  numerous  in  the  outer  nuclear  layer.  Their  nuclei  are 
among  those  of  the  inner  nuclear  layer.  The  nerve  cells  of  this  layer  are 
chiefly  small  bipolar  ganghon  cells  constituting  the  ganglion  retinae. 
The  dendritic  process  of  each  extends  into  the  outer  reticular  layer,  where 


Rod  cell. 
Stellate  ganglion  cell. 

Bipolar  cells. 


Wlii } 


0     ooxi^SS  "1    e^'e^^a- 


^t 


Amakrine  cells.   ^'M 
Centrifugal  nerve  fiber. 
Multipolar  ganglion  cell. 


Laj-er  of  rods  and  cones. 
Membrana  limitans 


f   Outer  nuclear  laver. 


(   Henle's  fibre  layer. 
Outer  reticular  laver. 


«''  Q^'"'T    ■     r   Inner  nuclear  layer. 


Inner  reticular  laver. 


Ganglion  cell  layer. 
Nerve  fiber  laver. 


Collateral. 
Fig.  412.— Diagram  of  Human  Retina.    Supporting  Substance  Red. 


Pyramidal  bases 
of  radial  fibers. 


by  forking  it  breaks  up  into  very  fine  fibers  parallel  \\dth  the  surface. 
They  form  a  subepithehal  feltwork  and  have  been  said  actually  to  anas- 
tomose. All  the  bipolar  ganghon  cells  send  their  longest  dendrite  between 
the  visual  cells  where  it  ends  in  a  little  thickening  near  the  membrana 
limitans.  The  neuraxons  of  the  bipolar  cells  pass  into  the  underlying 
inner  reticular  layer  and  there  break  up  in  fine  varicose  branches. 

The  inner  nuclear  layer  near  its  outer  boundary  contains  stellate  cells, 
sometimes  large,  which  send  many  dendrites  into  the  subepithelial  felt- 
work  where  they  anastomose.  Their  neuraxons  extend  horizontally,  and 
may  pass  inward  to  join  the  fibers  of  the  optic  ner^T  (which  is  denied  by 
some)  or  they  may  terminate  in  horizontal  branches  which  ascend  to  the 


360 


HISTOLOGY. 


bases  of  the  visual  cells  (Fig.  412,+).  Toward  the  inner  surface  of  the 
inner  nuclear  layer  there  are  large  ganghon  cells  which  send  branched 
processes  into  the  inner  reticular  layer.  Neuraxons  of  these  "amakrine 
cells"  have  not  been  found.  Some  fibers  extending  out  from  the  brain 
through  the  optic  nerve  terminate  in  free  endings  within  the  inner  nuclear 
layer. 

The  inner  reticular  layer  consists  of  a  very  fine  supporting  network, 
lodging  the  processes  of  the  bipolar  and  amakrine  cells,  together  with  the 
dendrites  of  large  multipolar  cells  of  the  ganglion  layer  beneath. 

The  ganglion  cell  layer  or  ganglion  0}  the  optic  nerve  consists  of  a 
single  layer  of  large  multipolar  cells  containing  Nissl  bodies.    Giant  forms 


Fiber  basket. 


Nucleated  part  of  the 
fiber. 


^^-   Basal  pyramid. 


*^    Precipitate. 

Fig.  413.— GoLGi   Preparation  ok  Radial   Fibers  in  a   Thick   Section  of  the   Human   Retina. 
The  fine  processes  of  the  fibers  in  the  outer  nuclear  layer  appear  as  a  compact  mass.    X  360. 

occur  at  quite  regular  intervals.  "Twin  cells"  have  been  described  as 
joined  by  a  short  bridge,  only  one  of  the  pair  having  a  neuraxon.  The 
branched  dendrites  of  these  ganghon  cells  extend  into  the  inner  reticular 
layer;  their  neuraxons  pass  toward  the  papilla  of  the  optic  nerve  and 
except  for  the  internal  limiting  membrane  which  covers  them,  they  form 
the  innermost  layer  of  the  retina.  Collaterals  have  been  detected  returning 
from  this  nerve  fiber  layer  to  branch  about  the  cell  bodies  of  the  ganghon 
layer.  The  nerve  fiber  layer  also  contains  the  centrifugal  fibers  which 
terminate  in  the  inner  nuclear  layer.  The  fibers  are  all  non-medullated. 
Summary.  The  elaborate  subdivision  of  the  retina  into  eleven  layers 
should  not  be  allowed  to  obscure  the  essential  features,  namely,  that  it 


MACULA   LUTEA   AND    FOVEA   CENTRALIS. 


^,61 


"^^^of'^&c 


■5 '3 


Zl  (U 

^  ?! 

2n, 

X 

^  0) 

.=  c3 

n 

m 

2"^ 

0 

t/i 

< 

.td  0 

> 

^>" 

> 

H 

X 

<u  v 

^   c  o 

<  5  c{ 


z 

rtl 

u 

-1- 

a 

a; 

a 

aj 

5    oS£<J 


o  ^ 


z  =  ^  ?; 
2  °oS 


u  -2 


\-.  ve^®'-o^e'mn''Oo- 

\ 

5 

.5 

c;.  414. — HORTZONTAI 

like  all  the  layers,  is 
section  as  minute  do 
granule  and  ganglion 

layer    — ' 
layet     — 
layei.  — 

1 

1 

5  z 


362  HISTOLOGY. 

consists  of  an  outer  pigmented  and  an  inner  visual  layer.  The  latter 
includes  an  outer  layer  of  visual  cells, — rod  cells  and  cone  cells.  The 
bipolar  cells  of  the  ganglion  retinae  receive  dendritic  fibers  which  have 
free  endings  between  the  visual  cells.  They  give  rise  to  branching  neu- 
raxons  which  communicate  with  the  ganglion  cells  of  the  optic  nerve. 
The  neuraxons  of  the  latter  converge  at  the  papilla  of  the  nerve  and  extend 
to  the  brain.  The  retina  also  receives  fibers  from  the  brain.  It  contains 
an  ectodermal  supporting  tissue,  blood  vessels  in  its  inner  layers,  and 
nerv-e  cells  perhaps  commissural,  the  significance  of  which  is  still  obscure. 

Macula  lutea  and  fovea  centralis.  When  vision  is  centered  upon  a 
particular  object  the  eyes  are  so  directed  that  the  image  of  the  object  falls 
upon  the  macula  lutea  or  yellow  spot  of  the  retina,  within  which  there  is  a 
depression,  the  fovea  centralis.  The  macula  receives  straight  slender 
fibers  from  the  papilla  of  the  optic  nerve  which  is  close  by  on  its  median 
side;  other  coarser  optic  fibers  diverge  as  they  pass  the  macula,  forming 
an  ellipse  around  it.  The  retinal  layers  of  the  macula  are  arranged  as 
show  in  Fig.  414.  At  its  border  the  number  of  rod  cells  diminishes  and 
within  the  macula  they  are  entirely  absent.  The  nuclei  of  the  numerous 
cone  cells,  which  are  here  somewhat  smaller  than  elsewhere,  form  an  inner 
nuclear  layer  of  twice  the  usual  thickness.  The  basal  portions  of  the 
cone  cells  make  a  broad  Henle's  fiber  layer  and  slope  away  from  the 
fovea.  The  bipolar  cells  of  the  ganglion  retinae  are  so  numerous  that 
their  nuclei  may  form  nine  rows.  The  ganglion  cells  of  the  optic  ner\-e 
are  also  abundant.  All  of  these  strata  become  thin  toward  the  fovea, 
the  deepest  part  of  which  contains  scarcely  more  than  the  cone  cells.  In 
some  individuals  the  slope  of  the  sides  of  the  fovea  is  less  steep  than  in  the 
figure;  its  depth  is  variable.  The  macula  and  fovea  are  saturated  with  a 
yellow  pigment  soluble  in  alcohol. 

Pars  ciliaris  retinae.  The  optic  nerve  fibers  and  their  ganglion  cells 
disappear  before  reaching  the  ora  serrata.  The  cone  cells  extend  further 
than  the  rods,  but  the  last  of  them  appear  to  lack  outer  segments.  By 
the  thinning  of  the  reticular  layer  the  nuclear  layers  become  confluent 
(Fig.  415).  Near  the  ora  serrata  large  clear  spaces  normally  occur  in 
the  outer  nuclear  layer  and  they  may  extend  into  the  deeper  layers  (Fig. 
415).  The  radial  sustentacular  cells  form  a  simple  columnar  epithelium 
as  the  other  layers  disappear,  and  they  constitute  the  visual  layer  of  the 
pars  ciliaris.  The  pigmented  epithehum  is  apparently  unmodified  as  it 
extends  from  the  optic  to  the  ciliary  portion.  Along  the  inner  surface  of 
the  visual  layer  of  the  ciliary  retina  the  cells  produce  horizontal  fibers 
closely  packed,  which  form  a  refractive  hyahne  membrane. 

Zonula  ciliaris.    Some  fibers  arising  from  the  pars  ciharis  immedi- 


jS     O 


'  Vacuole." 


Radial  fibers 
of  MiUler. 


Oft 


O    . 


Sx 

u 
< 

a  2 

5  '^ 

"^  u 

<> 

h 

< 


Pars  ciliaris  retinae.        ^ 


.-,6.1 


364 


HISTOLOGY. 


ately  in  front  of  the  ora  serrata  enter  the  vitreous  body,  but  a  much  larger 
number  pass  between  the  cihary  processes  to  the  lens.  They  are  attached 
to  the  borders  of  its  capsule,  overlapping  slightly  its  anterior  and  posterior 
surfaces.  Thus  they  form  the  zonula  ciliaris  [suspensory  ligament] 
which  holds  the  lens  in  place  (Fig.  410).  The  zonula  is  not  a  continuous 
layer,  nor  does  it  consist  of  two  laminae,  one  to  the  anterior  and  the  other 
to  the  posterior  surface  of  the  lens  with  a  space  between  them.  It  con- 
sists rather  of  numerous  bundles,  between  which  and  the  vitreous  body, 
and  among  the  bundles  themselves,  there  are  zonular  spaces  [canals  of  Petit] 
which  communicate  with  the  posterior  chamber. 

Optic  Nerve. 
In  its  intraorbital  portion  the  optic  nerve  is  surrounded  by  prolonga- 
tions of  the  meninges.    On  the  outside  is  the  dural  sheath,  consisting  of 


Hyaloid  membrane,   '' 
loosened. 


Central  artery. 
Fibers  of  the  lamina  cribrosa.      |      Cenjtral  vein. 


Retina.  — '  -  ^^^"^ 
Chorioid.  -i 

Sclera. 


Bundles  of  the  optic  nerve.  ^ 
Pial  sheath. 

Arachnoidal  sheath. 


Dural  sheath. 


Fig.  416. — LoNGiTi'DiNAi.  Section  of  the  Optic  Entrance  of  a  Human  Eye.    X  15. 
Above  the  lamina  cribrosa  is  seen  the  narrowing  of  the  optic  nerv^e,  due  to  its  loss  of  myelin.     The  central 
artery  and  vein   have  been  for  the  most  part  cut  longitudinally,  but  above  at  several  points  trans- 
^ersely. 

thick  outer  longitudinal  and  inner  circular  bundles  of  connective  tissue 
with  many  elastic  fibers.  Internally  it  is  connected  with  the  arachnoid 
layer  by  few  dense  strands  of  tissue,  and  the  arachnoid  joins  the  pial  sheath 
by  many  branched  trabeculae.  The  pia  surrounds  the  entire  nerve  and 
sends  anastomosing  layers  among  the  bundles  of  nerve  fibers.  The  latter 
are  slender  and  medullated  but  without  a  neurolemma;  they  are  supported 
by  long-rayed  neuralgia  cells  which  extend  between  the  individual  fibers, 
but  are  most  numerous  at  the  periphery  of  the  bundles  and  of  the  entire 


LENS. 


36  = 


nen-e.     Thus  the  optic   nen-e   differs   from   the   peripheral   nen^es    and 
resembles  a  cerebral  commissure. 

At  the  posterior  surface  of  the  eye  the  dura  blends  with  the  sclera. 
Continuous  with  both  is  the  dense  elastic  lamina  crihrosa  which  is  per- 
forated by  the  optic  nen'e  fibers.  The  chorioid  and  the  pia  are  also  in 
relation  with  the  lamina  (Fig.  416).  As  the  optic  nerv-e  passes  the  lamina, 
its  fibers  lose  their  myelin  and  radiate  into  the  nerve  fiber  layer  of  the  retina. 
The  central  artery  and  vein  of  the  retina  enter  the  optic  nerve  in  its  distal 
half,  and  appear  at  the  fundus  of  the  eye  in  the  center  of  the  optic  papilla. 
Their  branches  spread  in  the  inner  layers  of  the  retina,  outside  of  the 
membrana  limitans  interna. 

Lexs. 
The  lens  is  a  biconvex  structure  having  an  anterior  and  a  posterior 
pole,  and  a  vertical  equatorial  plane.  It  is  enclosed  in  a  thick  transparent 
elastic  capsule  which  is  6.5-25  />-  thick  in  front,  and  2-7  u  thick  behind. 
Within  the  capsule  the  anterior  surface  of  the  lens  is  formed  by  the  lens 
epithelium,  a  single  layer  of  cells  2.5  fj.  thick  at  the  pole  but  becoming  taller 


Fig.  417. — Lexs  Fibers  of  a  New-born 
Infant. 

A,  Isolated  lens  fibers,  three  with  smooth, 
one  with  dentate  borders.  /,  240.  B, 
Human  lens  fibers  cut  transversely  ;  c, 
section  through  club  shapec^  ends. 
X  560. 


a 


A    n^m-) 


■-rP^:^ 


Fig.  418.— Capsule  and  Epithelium  of  a  Lens  of 
Adult  Man. 

C,  Inner  aspect.  D,  Lateral  aspect,  from  a  meridional 
section  through  the  equator  of  the  lens;  i,  cap- 
sule; 2,  epithelium  ;  3,  lens  fibers.     X  240, 


at  the  equator.  There  they  are  continuous  with  the  elongated  lens  phers 
of  the  posterior  layer,  which  collectively  are  called  the  substantia  lentis. 
New  fibers  are  formed  by  the  mitosis  of  cells  at  the  periphery  of  the  pos- 
terior layer.  The  lens  fibers  are  generally  six-sided  prisms,  somewhat 
enlarged  at  one  or  both  ends.  The  central  fibers  have  lost  their  nuclei; 
their  boundaries  are  wavy  or  notched.     These,  which  were  the  first  to 


366  Histology. 

form,  constitute  a  dense  mass,  the  niideiis  0}  the  lens.  The  outer  fibers 
of  the  cortical  substance  are  softer.  They  have  smooth  borders,  and  nuclei 
which  are  chiefly  in  the  equatorial  plane.  Their  protoplasm  is  transformed 
into  a  clear  fluid  substance,  said  to  be  chiefly  a  globulin.  The  fibers  are 
united  to  one  another  by  a  small  amount  of  cement  substance,  which  is 
more  abundant  at  the  poles;  after  maceration  of  the  lens  it  generally  radiates 
from  either  pole,  forming  a  stellate  figure  around  each.  These  have  three 
rays  in  older  embryos  and  ordinarily  nine  rays  in  the  adult.  The  lens 
fibers  all  run  in  the  meridional  direction  from  the  anterior  stellate  rays  to 
the  posterior.  The  nearer  the  anterior  pole  they  arise,  the  further 
from  the  posterior  pole  they  terminate,  and  vice  versa,  since  no  fiber  is 
long  enough  to  extend  from  one  pole  to  the  other.  The  fibers  of  the  cor- 
tical substance  are  said  to  form  about  2000  radial  lamellae  comparable 
with  the  segments  of  an  orange.  Owing  to  the  differences  in  consistency 
of  fibers  of  various  ages,  concentric  lamellae  may  be  separated  in  hardened 
lenses. 

Vitreous  Body. 
The  corpus  vitreum  consists  of  the  fluid  vitreous  humor  and  loose 
fibrous  strands  of  stroma.  Although  some  recent  pathological  cases  sug- 
gest that  the  latter  are  arranged  like  the  septa  of  an  orange,  it  has  not  been 
estabhshed  that  they  have  any  definite  arrangement.  The  cells  of  the 
vitreous  body  are  round  forms,  probably  leucocytes,  and  stellate  or  spindle 
shaped  forms  representing  the  connective  tissue  which  invaded  the  vitreous 
body  with  the  blood  vessels.  The  latter  have  atrophied  and  been  resorbed. 
Opaque  flakes  which  occur  normally  and  float  into  the  field  of  vision 
as  "muscae  vohtantes,"  have  been  ascribed  to  fragments  of  degenerated 
tissue  ;  vacuolated  degenerating  cells  have  been  observed.  Crystals 
may  form  and  settle  in  the  lower  part  of  the  bulb.  The  vitreous  body  is 
bounded  by  a  very  resistant  thick  fibrous  layer  which  does  not  justify  the 
term  hyaloid  membrane. 

Tunica  Vasculosa. 
Chorioid.  Between  the  sclera  and  the  chorioid  there  is  a  loose  tissue 
containing  many  elastic  fibers  and  branched  pigment  cells,  together  with 
flat  non-pigmented  cells.  In  separating  the  sclera  from  the  chorioid  this 
layer  is  divided  into  the  lamina  fusca  of  the  sclera  and  the  lamina  supra- 
chorioidea.  Internal  to  the  latter  is  the  lamina  vasculosa  which  forms  the 
greater  part  of  the  chorioid.  It  contains  many  large  blood  vessels  imbedded 
in  a  loose  elastic  connective  tissue,  some  of  its  cells  being  branched  and 
pigmented;  others  without  pigment  are  flat  and  arranged  in  layers  surround- 


CHORIOID. 


567 


ing  the  vessels.  A  thin  inner  layer  of  blo.od  vessels,  the  lamina  choriocapil- 
laris,  consists  of  a  very  close  network  of  wide  capillaries.  The  choriocapil- 
laris  is  separated  from  the  pigmented  epithehum  of  the  retina  by  a 
structureless  elastic  lamella  which  may  be  2  n  thick.  This  lamma  hasalis 
shows  the  imprint  of  the  polygonal  retinal  cells  on  its  inner  surface  and  is 
associated  with  fine  elastic  networks  toward  the  choriocapillaris. 

Between  the  vascular  lamina  and  the  choriocapillaris  there  is  a  boundary 
layer  of  fine  elastic  networks  generally  without  pigment.  Here  in  ruminants 
and  horses  there  are  many  wav\'  bundles  of  connective  tissue  which  give  to  the 
eyes  of  those  animals  a  metallic  luster.  Such  a  layer  is  known  as  the  tapetum 
fibrosum.  The  similarly  iridescent  tapetum  cellulosnm  of  the  carnivora  is 
formed  of  several  layers  of  flat  cells  which  contain  numerous  fine  crvstals. 


Cross  and  longitudinal 
sections  of  bundles 
of  scleral  fibers. 

Lamina  supra- 
chorioidea. 


Lamina  vasculosa. 


Boundary  zone. 
Choriocapillaris. 
Basal  membrane. 
Pigment  layer  of  the 
retina. 


Fig.  419.— Vertical  Section  through  a  part  of  the  Human  Sclera  and  the  entire  thickness 

OF  the  Chorioid.    X  100. 
g,  Large  vessels  ;  p,  pigment  cells  ;  c,  cross  sections  of  capillaries. 


The  ciliary  body  encircles  the  eye  as  a  muscular  band,  attached  to  the 
inner  surface  of  which  there  are  from  70  to  80  meridional  folds,  the  ciliary 
processes  (Fig.  410).  The  equator  of  the  eye  is  vertical,  hke  that  of  the 
lens,  and  the  meridians  are  antero-posterior.  The  processes  begin  low  at 
the  ora  serrata  and  rise  gradually  to  a  height  of  i  mm.,  terminating  abruptly 
near  the  border  of  the  lens.  Each  process  consists  of  fibrillar  connective 
tissue  containing  numerous  elastic  fibers  and  blood  vessels,  and  is  bounded 
toward  the  pars  ciliaris  retinae  by  a  continuation  of  the  lamina  basahs 
which  forms  intersecting  folds.  The  cihary  processes,  which  are  com- 
pressible, may  serve  to  prevent  the  increase  of  intraocular  pressure  during 
the  contraction  of  the  cihary  muscle.     The  ciliary  muscle  is  a  band  of 


368 


HISTOLOGY. 


smooth  muscle  fibers  about  3  mm.  broad  and^o.8  mm.  thick  anteriorly; 
it  arises  beneath  the  sinus  venosus  of  the  sclera  and  tapers  toward  the  ora 
serrata  (Fig.  410).     It  consists  of  two  sets  of  fibers,  the  meridional  and 

circular.  The  meridional  fibers 
as  seen  in  section  (p.  356), 
form  a  triangular  group  con- 
verging toward  the  sinus  veno- 
sus. Their  numerous  outer- 
most bundles  mixed  with  elas- 
tic tissue  are  apphed  to  the 
scleral  surface.  Anteriorly 
the  bundles  become  gradually 
shorter  and  more  radially 
placed  so  that  those  in  the 
front  of  the  muscle  are  perpen- 
dicular to  the  sclera.  The 
radial  fibers  are  classed  as  a 
separate  group  by  Professor 
Stohr.  The  circular  fibers 
which  vary  in  number  indifferent  individuals  form  that  part  of  the 
ciliary  muscle  which  is  nearest  to  the  equator  of  the  lens. 

The  iris  consists  of  its  stroma  anteriorly  and  the  pars  iridica  retinae 


Fig.  420.— a.  From  a  Teased  Preparation  of  a  Hu- 
man Chorioid.  X  240.  p,  Pigment  cells  ;  e,  elastic 
fibers  ;  k,  nucleus  of  a  flat  non-pigmented  cell ;  the 
cell  body  is  invisible. 

B,  Portion  of  a  Human  Choriocapillaris  and  the 
Adherent  Lamina  Basalis.  X  240.  c,  Wide  capil- 
laries, some  of  which  contain  (b)  blood  corpuscles; 
e,  lamina  basalis,  showing  a  fine  "  lattice  work.'' 


Mesenchymal 
epithelium. 


Loose  connective 
tissue. 


Vascular  laver. 


Spindle  cell  layer. 


^±_  Pars  iridica 
retinae. 


Fig.  421 —Vertical  Section  of  the  Pupillary  Portion  of  a  !.    ...  ...  .      ,^  100.     About  one- 
fifth  of  the  eniire  width  of  the  iris  is  shown, 
g,  Blood  vessel,  with  thick  connective  tissue  sheath;  m,  sphincter  pupillae  muscle  cut  transversely;  p, 

pupillary  border  of  the  iris. 


posteriorly,  and  is  covered  by  the  mesenchymal  epithelium  of  the  chambers 
of  the  eye.  The  anterior  epithelium  is  a  simple  layer  of  flat  polygonal 
cells  [unfortunately  named  endothelium].     The  stroma  consists  anteriorly 


IRIS.  369 

of  a  network  of  stellate  cells  in  part  pigmented.  It  is  followed  by  a  vas- 
cular layer  of  fine  loose  connective  tissue  with  few  elastic  fibers.  Its 
stellate  cells,  which  in  blue  eyes  are  not  pigmented,  form  elongated  polyg- 
onal meshes.  The  vessels  are  radial,  and  have  a  thick  connective  tissue 
externa  but  a  very  weak  circular  musculature.  Among  the  vessels  near 
the  free  border  of  the  iris,  there  are  smooth  muscle  fibers  which  form  a 
band  i  mm.  wide  encircling  the  pupil.  This  is  the  sphincter  muscle  of 
the  pupil.  A  few  radial  muscle  fibers  also  occur  among  the  vessels. 
The  dilator  muscle  of  the  pupil  is  behind  the  vascular  layer.  It  is  a  con- 
tinuous layer  of  radially  arranged  smooth  muscle  fibers,  beginning  near  the 
pupil  and  extending  to  the  ciliary  body.  The  contractile  portion  of  the 
spindle  shaped  muscle  cells  forms  a  membrane-like  layer  resting  against 
the  pars  iridica  retinae,  with  which  the  pigmented  nucleated  portion  of 
the  cells  seems  to  unite.  These  muscle  cells  have  been  thought  to  arise 
from  the  outer  layer  of  the  retinal  cup.  Except  in  albinos  both  layers 
of  the  retina  are  here  heavily  pigmented,  and  apart  from  their  embr}-o- 
logical  development  they  would  be  regarded  as  a  single  layer. 

Tunica  Fibrosa. 

The  sclera  consists  of  interwoven  bundles  of  connective  tissue,  chiefl}- 
meridional  and  longitudinal.  Elastic  tissue  accompanies  the  bundles  and 
is  especially  abundant  at  the  insertions  of  the  ocular  muscles.  The  flat 
irregular  cells  of  the  connective  tissue  are  surrounded  by  tissue  spaces  as 
in  the  cornea.  Xext  to  the  chorioid,  the  sclera  forms  a  pigmented  lamina 
fusca  which  has  already  been  described.  The  sclera  becomes  thinner 
anteriorly  where  it  is  absolutely  continuous  with  the  transparent  cornea. 
The  corneal  boundary  is  oblique,  being  bevelled  at  the  expense  of  its 
anterior  surface. 

The  cornea  (Fig.  422)  consists  of  an  outer  epithelium,  external 
basal  membrane,  substantia  propria,  internal  basal  membrane,  and  mesen- 
chymal epithelium  bounding  the  anterior  chamber.  The  corneal  epithe- 
lium, about  .03  mm.  thick,  is  stratified  and  consists  of  a  basal  layer  of 
clearly  outlined  columnar  cells  followed  by  three  or  four  rows  of  cuboidal 
cells  and  several  layers  of  flattened  superficial  cells.  The  outer  cells 
retain  their  nuclei.  Peripherally  the  epithehum  is  continuous  with  that 
of  the  conjunctiva  bulbi.  The  anterior  basal  membrane  [Bowman's]  is 
an  almost  homogeneous  layer,  sometimes  as  much  as  .01  mm.  thick. 
Superficially  it  connects  with  the  epithelial  cells  by  bands  and  processes. 
Beneath  it  blends  with  the  substantia  propria,  of  which  it  is  a  modification. 
Since  it  is  not  formed  of  elastic  substance  the  name  "anterior  elastic  mem- 
brane" is  not  justified. 
24 


37° 


HISTOLOGY. 


The  substantia  propria  consists  of  delicate  straight  connective  tissue 
fibrils  which  are  united  in  bundles  of  an  almost  uniform  thickness  by  a 


Epithelium. 
Anterior  basal  membrane. 


Substantia  propria.   . 


''%;?.V«l 


'«j#»»^iiiif*_ 


Posterior  basal  membrane.    _ 


Mesenchymal  epithelium.     '  '\■^'\v^^Slw<vvv;"^'^ 

Fig.  422. — Vertical  Section  of  a  Human  Corne,\.    X  100. 


Corneal  canaliculus.  Corneal  space. 


Corneal  cells. 


Fig.  423.— Corneal  Spaces  and  Canaliculi  (in  F'ig.  424.— Corneal  Cells  from  a  Horizontal 

White)    from  a    Horizontal   Section  of  Section    of    the    Cornea    oi'    a    Rabbit. 

THE  Cornea  of  an  Ox.    Silver  preparation.  X  240. 
X  240. 

(fluid?)  interfibrillar  cement.     The  bundles  are  cemented  together,  form- 
ing superposed  flat  lamellae  parallel  with  the  surface.     The  layers  are 


CORNEA.  371 

connected  by  an  interlamellar  cement  substance,  and  by  occasional  oblique 
fiber-bundles.  The  latter  so-called  arcuate  fibers  are  to  be  found  especially 
between  the  anterior  layers.  In  the  cement  substance,  there  is  a  system  of 
branched  canaliculi,  dilated  in  places  to  form  oval  spaces.  The  latter  are 
between  lamellae  but  the  canaliculi  extend  among  their  constituent  fiber- 
bundles.  Within  the  spaces  there  are  fiat  stellate  anastomosing  cells,  the 
branches  of  which  extend  into  the  canals  and  tend  to  unite  with  those  of 
neighboring  cells  at  right  angles.  The  cells  and  their  processes  are  more 
or  less  surrounded  by  tissue  fluid.  Leucocytes  enter  the  canals  and  are 
normally  found  in  the  cornea;  if  the  cornea  is  inflamed  they  become  abun- 
dant.    Blood  vessels  and  lymphatic  vessels  are  absent. 

The  posterior  basal  membrane  [Descemet's  membrane]  is  a  clear 
elastic  lamina,  6  jj-  thick.  Its  inner  surface  in  the  adult  shows  hemispher- 
ical elevations.  The  mesenchymal  epithelium  is  a  simple  layer  of  flat 
polygonal  cells.  The  iris  sends  connective  tissue  prolongations  over  the 
peripheral  part  of  the  inner  corneal  surface.  Collectively  they  are  called 
the  ligamentum  pectinatum  of  the  iris.  As  compared  with  those  of  the  ox 
and  horse,  in  man  they  are  rudimentary. 

Blood  Vessels. 

The  central  vessels  of  the  retina  supply  a  part  of  the  optic  nerve  and 
the  retina;  the  ciliary  vessels  supply  the  rest  of  the  eye.  These  two  sets 
of  vessels  anastomose  with  one  another  only  at  the  entrance  of  the  optic 
nerve  (Fig.  425). 

The  ciliary  arteries  are  (i)  the  short  posterior  ciliary  arteries  to  the 
chorioid;  and  (2)  the  long  posterior  ciliary  arteries  which  with  (3)  the 
anterior  ciliary  arteries  supply  chiefly  the  ciliary  body  and  iris.  The  three 
groups  will  be  considered  in  turn. 

1.  After  supplying  the  posterior  half  of  the  surface  of  the  sclera^ 
some  twenty  branches  of  the  short  posterior  cihary  arteries  penetrate 
the  sclera  around  the  optic  nerve.  They  form  the  capillaries  of  the 
lamina  choriocapillaris.  At  the  entrance  of  the  optic  nerve  they  anasto- 
mose with  branches  of  the  central  artery  of  the  retina  (c)  and  thus  form 
the  circulus  arteriosus  nervi  optici.  At  the  ora  serrata  they  anastomose 
with  recurrent  branches  of  the  long  posterior  cihary  and  the  anterior 
ciliary  arteries. 

2.  The  two  long  posterior  ciliary  arteries  also  penetrate  the  sclera 
near  the  optic  nerve  (j).  They  pass,  one  on  the  nasal  and  the  other  on  the 
temporal  side  of  the  eye,  between  the  chorioid  and  sclera  to  the  ciliary  body. 
There  each  artery  divides  into  two  diverging  branches  extending  along  the 


o/- 


HISTOLOGY. 


ciliary  border  of  the  iris.  By  the  anastomosis  of  these  four  branches  a 
vascular  ring  is  formed,  the  circnlus  iridis  major  (2),  from  which  numerous 
branches  proceed  to  the  ciliary  processes  (j)  and  to  the  iris  (4).     Near 


Branches  Branches 
to  the         to  the 
Sinus     corneal  conjunctiva 
venosus    border.       bulbi. 


Cornea.  sclerae.      ^^  V 


Connection  \vith  the  lamina  choriocapillaris. 


-cilians  anterior. 


\'enous    1  Episcleral 

I  branches  of  the 

[  anterior 

-Arterial    J  ciliarv  vessels. 


Capillaries  of  the  lamina  choriocapillaris 


\'ena  vorticosa. 


\"enous  1   Episcleral    branches 
.Arterial         j  of  the  short  posterior 
ciliarv  vessels. 


.       •      S-ciliaris  posterioris  brevi 


Outer) 
Inner  j 


vessels  of  the  sheath. 


Short'"posterior  ciliary  arteries 


A'ena     .Arleria 


centralis  retinae. 


Fig.  425. — Bi.ooi)  \'K.stELs  of  thh  Eyk.     (After  Leber.) 

The  retina,  optic  ?ierve.  and  tunica  fibrosa  are  stippled  :  the  tunica  vasculosa  is  blank.     V.  Connection  of 
the  anterior  ciliarv  artcr\  w  illi  the  circnlus  iridis  major. 


the  pupillary  border  of  the  iris  the  arteries  form  an  incomplete  ring,  the 
circnlus  iridis  minor. 

3.  The  anterior  ciliary  arteries  ari.sc  from  those  supplying  the  recti 


BLOOD    VESSELS    OF   THE    EYE.  373 

muscles,  penetrate  the  sclera  near  the  cornea,  and  in  part  join  the  circulus 
iridis  major,  in  part  supply  the  ciliary  muscle,  and  in  part  through  recurrent 
branches,  connect  with  the  lamina  choriocapillaris.  Before  penetrating 
the  sclera  the  anterior  ciliary  arteries  give  off  posteriorly  branches  for  the 
anterior  half  of  the  sclera,  and  anteriorly  branches  for  the  conjunctiva 
bulbi  and  the  corneal  border.  The  cornea  itself  is  without  vessels,  but  at 
its  border,  between  the  anterior  lamellae  of  the  substantia  propria,  there 
are  terminal  loops. 

The  veins  generally  proceed  toward  the  equator,  uniting  in  4 
(less  often  in  5  or  6)  venae  vorticosae.  These  pass  directly  through  the 
sclera  and  empty  into  one  of  the  ophthalmic  veins.  Besides  the  venae 
vorticosae  there  are  small  veins  accompanying  the  short  posterior  and  the 
anterior  ciliary  arteries.  The  short  ciliary  veins  receive  branches  from 
the  ciliary  muscle,  the  episcleral  vessels,  the  conjunctiva  bulbi  and  the 
periphery  of  the  cornea.  The  episcleral  veins  also  connect  with  the  venae 
vorticosae.  Within  the  sclera  near  the  cornea  there  is  a  circular  vein 
receiving  small  branches  from  the  capillaries  of  the  ciliary  muscle.  This 
sinus  venosus  sclerae  [canal  of  Schlemm]  connects  with  the  anterior  ciliary 
veins. 

After ia  centralis  retinae.  From  15  to  20  mm.  from  the  eye  the  central 
artery  of  the  retina  passes  to  the  axis  of  the  optic  nerve  and  proceeds  to 
the  optic  papilla.  There  it  divides  into  two  branches  directed  upward 
and  downward  respectively,  and  these  by  further  subdivision  supply  the 
entire  pars  optica  retinae.  The  branches  are  chiefly  in  the  inner  layers 
but  may  extend  into  the  outer  reticular  layer;  they  are  absent  from  the 
fundus  of  the  fovea  centralis.  Within  the  optic  nerve  the  artery  sends  out 
numerous  little  branches  which  anastomose  with  small  vessels  which  have 
entered  the  sheaths  from  the  surrounding  fat;  and  also  with  branches  of 
the  short  posterior  ciliary  arteries  (Fig.  425,  b). 

The  central  vein  of  the  retina  receives  two  main  branches  at  the  optic 
papilla  and  follows  the  artery  along  the  axis  of  the  optic  nerve. 

Chambers  .^jstd  Spaces  of  the  Eye. 
The  eye  contains  no  lymphatic  vessels  but  is  provided  with  communi- 
cating tissue  spaces,  bounded  by  mesenchymal  cells  or  epitheha.  These 
include  the  canaliculi  of  the  cornea  and  sclera;  and  the  anterior  chamber 
of  the  eye  which  through  the  capillary  interval  between  the  lens  and  iris 
connects  with  the  posterior  chamber,  and  the  latter  is  prolonged  into  the 
zonular  spaces.  Irregular  extensions  of  the  anterior  chamber,  associated 
with  the  pectinate  ligament  of  the  iris,  are  called  spaces  of  the  angle  of  the 
iris  [spaces  of  Fontana].     They  are  but  slightly  developed  in  man.     Pos- 


374  HISTOLOGY. 

teriorly  the  tissue  spaces  include  the  hyaloid  canal  of  the  vitreous  body; 
the  very  narrow  perichorioideal  space  between  the  chorioid  and  sclera; 
the  subdural  and  subarachnoid  spaces  of  the  optic  sheaths,  named  the 
intravaginal  spaces;  and  finally  the  interjascial  space  [of  Tenon]  which 
surrounds  most  of  the  sclera  and  is  prolonged  as  a  supradural  space  around 
the  optic  nerve.  These  spaces  may  be  filled  from  the  subarachnoid  of 
the  brain.  They  contain  a  "filtrate  from  the  vessels."  The  interfascial 
and  perichorioideal  spaces  hold  but  little  fluid;  acting  as  bursae,  they  may 
facilitate  the  movements  of  the  eye. 

Nerves. 
Apart  from  the  optic  nerve,  the  eye  is  supplied  by  the  sliort  ciliary 
nerves  from  the  cihary  ganghon,  and  the  long  ciliary  nerves  from  the  naso- 
ciliary branch  of  the  ophthalmic  nerve.    The  ciliary  nerves  penetrate  the 

sclera  near  the  optic  nerve  and 
send  branches  containing  gang- 
lion cells  to  the  vessels  of  the 
chorioid.  The  nerves  pass  for- 
ward between  the  chorioid  and 
sclera  to  the  ciliary  body,  where 
they  form  a  circular  ganglionated 
plexus,     the    plexus    gangliosus 

Fig.  426.-FROM  A  Section  OF  THE  Human  Cornea.        cUiarls.       ItS    branchcS  extend   tO 

n,    A  dividing    nerve  p^enXating  the  anterior   basal  (l)   ^^6    clllary    body,   (2)     the    iris 

membrane;    s.   subepithelial  plexus  beneath  the  or\A    ( 'i\  fVio  ,-/->t-t-ioo 

cylindrical  cells;    a,  fibers  of    the  intraepithelial  '^^^'"^   \6)   '-^^^  LOXliea. 
plexus  ascending  (5if/z(;'i?«2  the  epithelial  cells.  rv^-t  r     ^i  -i* 

ihe   nerves    of    the    ciliary 

body  form  a  dehcate  network  on 

its  scleral  surface ;  they  supply  its  muscle  fibers  and"  those  of  the  vessels 

with  slender  motor  endings,  and  between  the  ciliary  muscle  bundles  they 

have  branched  free  endings,  perhaps  sensory. 

The  medullated  nerves  of  the  iris  lose  their  myelin  and  form  plexuses 
as  they  pass  toward  the  pupillary  margin.  A  sensory  plexus  is  found  just 
beneath  the  anterior  surface,  and  motor  fibers  supply  the  sphincter, 
dilator  and  vascular  muscles.  The  existence  of  ganglion  cells  in  the  human 
iris  is  denied. 

The  nerves  of  the  cornea  enter  it  from  the  plexus  annularis  in  the 
sclera  just  outside.  The  annular  plexus  also  sends  fibers  into  the  conjunc- 
tiva, where  they  end  in  networks,  and  in  bulbous  corpuscles  (Fig.  128, 
p.  106)  situated  in  the  connective  tissue  close  to  the  epithelium.  Such 
corpuscles  may  be  found  i  or  2  mm.  within  the  corneal  margin.  The 
corneal  nerv^es  become  non-medullated  and  form  plexuses  between  the 


EYELIDS.  375 

lamellae  throughout  the  stroma.     They  extend  into  the  epithehum  and 
there  form  a  very  delicate  plexus  with  free  intercellular  endings. 

Eyelids. 

The  eyelids  or  palpebrae  (Fig.  427)  are  covered  with  thin  skin  pro- 
vided with  iine  lanugo  hairs;  small  sweat  glands  extend  into  the  corium. 
The  latter  contains  pigmented  connective  tissue  cells,  which  are  rare  else- 
where in  the  corium.  The  subcutaneous  tissue  is  very  loose,  having 
many  elastic  fibers  and  few  or  no  fat  cells.  Near  the  edge  of  the  Hd  there 
are  two  or  three  rows  of  large  hairs,  the  eyelashes  or  cilia,  the  roots  of  which 
extend  obliquely,  deep  into  the  corium.  Since  they  are  shed  in  from  100 
to  150  days  they  occur  in  various  stages  of  development.  They  are  pro- 
vided with  small  sebaceous  glands,  and  the  ciliary  glands  [of  Moll]  open 
close  beside  or  into  their  sheaths.  The  ciliary  glands  are  modified  sweat 
glands  with  simpler  coils  which  may  show  successive  constrictions;  "a 
branching  of  the  tubules  has  been  observed." 

The  central  portion  of  the  eyehds  is  muscular.  Beneath  the  sub- 
cutaneous tissue  there  are  striated  bundles  of  the  orhiciilaris  palpebrarum 
extending  lengthwise  of  the  lid.  A  subdivision  of  this  muscle  found  behind 
the  roots  of  the  ciHa  is  called  the  musculus  ciliaris  Riolani.  Posterior  to 
the  orbicularis  muscle  are  found  the  terminal  radiations  of  the  tendon  of 
the  levator  palpebrae.  A  part  of  these  are  lost  in  connective  tissue;  another 
part  associated  with  smooth  muscle  fibers,  is  inserted  into  the  upper  border 
of  the  tarsus  and  forms  the  superior  tarsal  muscle.  This  occurs  in  the  upper 
lid,  but  correspondingly  in  the  lower  lid  the  radiations  from  the  inferior 
rectus  muscle  contain  smooth  muscle  fibers,  forming  the  inferior  tarsal 
muscle. 

The  inner  portion  of  the  Kds  consists  of  the  conjunctival  epithehum 
and  the  underlying  connective  tissue  including  the  tarsus.  This  is  a  plate 
of  dense  connective  tissue  which  gives  firmness  to  the  lid.  It  begins  at  the 
free  edges  and  extends  over  the  adjacent  two-thirds  of  the  lid  close  to  the 
conjunctiva.  Imbedded  in  its  substance  in  either  Hd  there  are  about  30 
tarsal  glands  [Meibomian],  which  consist  of  a  wide  excretory  duct  opening 
along  the  palpebral  border  and  of  small  acini  with  short  stalks  which  enter 
it  from  all  sides.  In  structure  they  resemble  sebaceous  glands.  At  the 
upper  end  of  the  tarsus  and  partly  enclosed  in  its  substance,  there  are 
branched  tubular  accessory  lachrymal  glands.  They  occur  chiefly  in  the 
medial  (nasal)  half  of  the  lid. 

The  tunica  propria  of  the  palpebral  conjunctiva  contains  plasma  and 
lymphoid  cells;  the  latter  invade  the  epithehum  beneath  which  in  some 
animals  they  form  nodules.     The  stratified  epithelium  of  the  skin  gradually 


376 


HISTOLOGY. 


changes  to  that  of  the  conjunctiva,  which  has  several  basal  layers  of  cu- 
boidal  cells  and  a  superficial  layer  of  short  columnar  cells.     The  latter  are 


Superior 

tarsal  Radiations 

muscle.  from  the  tendon  Orbicularis 

Conjunctiva.       .  of  the  levator  palpebrae.     palpebrarum.  Skin. 


Epithe- 
lium. 


Tunica, 
propria. 


Arcus  -  - 
tarseus 
e.vternus. 


Papillae. 


^^^^^: 


;i% 


Epider- 
mis. 


Coriuni. 


Stratum 
subcuta- 
ueum. 


Sweat  gland. 


Oblique  section 
of  a  hair  sheath. 


Tarsal  .e-land. 


^SBM' 


Arcus  tarseus. 


V^f^>:-.*^j 


"'  ■tf^/'         Cross  section  of  the 

'~- ^ *       bundles  of  the 

orbicularis 
palpebrarum. 


Posterior  ed^t- 
of  the  lid 


Musculus 
ciliaris  Riolani. 


Part  of  a 
ciliary  gland. 


C  iliuni. 


Fin.  427. — Sagitt.al  Skction  of  the  Uppkr  Lid  of  a  Six  Months  Child.     The  outlet  of  the  tarsal 
gland  was  not  in  the  plane  of  section.     X  15. 


covered  by  a  thin  cuticula,  and  goblet  cells  are  found  among  them.     The 
transition  from  the  superficial  squamous  cells  to  the  columnar  form  may 


LACHRYMAL    GLANDS.  377 

occur  at  the  posterior  edge  of  the  Hd  or  quite  high  on  the  conjunctival  sur- 
face. Toward  where  the  palpebral  conjunctiva  arches  to  form  that  of  the 
bulb,  its  epithelium  is  so  folded  that  in  sections  it  may  seem  to  form  glands. 

The  conjunctiva  bulbi  is  similar  to  that  of  the  hd.  Its  outer  epithehal 
cells,  however,  become  squamous  toward  the  cornea  and  over  the  exposed 
portion  of  the  eye.  Its  basal  cells  contain  pigment,  except  in  the  European 
races.  The  yellow  color,  often  most  pronounced  near  the  medial  border 
of  the  cornea  and  known  as  Pinguecula,  is  said  not  to  be  due  to  fat  or  to  an 
epithelial  pigment;  it  accompanies  a  thickening  of  the  connective  tissue 
layer.  The  tunica  propria  forms  well  marked  papillae  near  the  cornea. 
Its  lymphocytes  may  form  nodules,  as  many  as  twenty  having  been  found 
in  the  human  conjunctiva  bulbi.  Occasional  mucous  glands  occur.  (It 
may  be  noted  that  the  entire  anterior  covering  of  the  bulb  of  the  eye  is 
named  by  some  the  conjunctiva  bulbi,  which  accordingly  is  divided  into  the 
conj.  sclerae  and  the  conj.  corneae.) 

At  the  medial  angle  of  the  lids  there  is  a  thin  fold  of  connective  tissue 
covered  with  stratified  epithelium;  this  plica  semilunaris  is  a  rudimentary 
third  hd.  The  nodular  elevation  of  tissue  at  the  medial  angle,  the  carun- 
cula  lacrimalis,  resembles  skin  except  that  a  stratum  comeum  is  lacking; 
it  contains  fine  hairs,  sebaceous  and  accessory  lachrymal  glands,  and  in  its 
middle  part  small  sweat  glands. 

The  blood  vessels  of  the  lids  proceed  from  branches  approaching  the 
lateral  and  medial  angles  of  the  eye.  They  form  an  arch,  the  arciis  tar- 
seus  externus,  at  the  upper  border  of  the  tarsus  (Fig.  427).  They  extend 
into  the  conjunctiva  bulbi,  and  near  the  margin  of  the  cornea  they  pass 
inward  to  unite  with  the  anterior  ciliary  vessels  (Fig.  425).  The  lymphatic 
vessels  form  a  close  network  beneath  the  palpebral  conjuncti^-a  and  a  loose 
one  in  front  of  the  tarsus.  WTiether  the  lymphatic  vessels  of  the  conjimc- 
tiva  bulbi  end  blindly  toward  the  cornea  or  connect  with  the  canaliculi,  has 
not  been  determined.  The  nen-es  form  a  very  thick  plexus  in  the  tarsus 
and  supply  the  tarsal  glands.  There  are  free  endings  in  the  conjunctival 
epithelium,  and  bulbous  corpuscles  in  the  connective  tissue  beneath. 

LACHRYiLAL    GlAXDS. 

The  lachrymal  glands  are  compound  tubular  glands  with  several 
excretory  ducts.  These  are  lined  with  a  double  row  of  epithelial  cells,  the 
superficial  layer  being  columnar.  The  excretory  ducts  pass  gradually 
into  long  intercalated  ducts  with  a  low  epithehum.  These  terminate  in 
tubules  presenting  two  forms  of  cells  and  surrounded  by  a  membrana 
propria.  The  cells  of  one  form  are  tall  when  filled  with  secretion,  which 
occupies  the  superficial  half  of  the  cell;  when  empty  they  are  shorter. 


378  HISTOLOGY. 

The  cells  of  the  other  form  are  low  when  full  of  secretion,  which  gathers  in 
a  large  round  mass,  leaving  only  a  thin  basal  layer  of  protoplasm.  Inter- 
cellular secretory  capillaries  and  secretory  granules  have  been  demon- 
strated. Between  the  gland  cells  and  the  basement  membrane  there  are 
occasional  flat  cells,  a  continuation  of  the  deeper  layer  of  the  epithelium 

of  the  duct.     The  blood  ves- 
B  ,  :v,.  ^^^^  ^^^  nerves  are  similar 

to  those  of  the  oral  glands. 


"^^'^  v*;v^!*^V;,f  The      two     lachrymal 

ducts  which  at  the    medial 
'4.  ,        -  .     -  v^     .        angle  of    the     eye  connect 


A 


b — - 

i 


r\ 


^«JIL?-' 


.    ^^,      ^,  , ^  \^  with  thenasolacJirymal  duct, 

•f* ;  :f//'^i^        ^        must  not  be  mistaken  for 
■!;;i\°|^B^'     >  the  excretory  ducts  of  the 

'<^^rX   ■  lachrymal  glands.   The  for- 

mer   consist     of    stratified 
epithelium  with   squamous 


Fig.  428. — From    a    Skction    of    a     Human    Lachrymal 
Gland.    X  420. 

A,  Gland  body ;  a,  tubule  cut  across  ;    a',  group  of  tubules  cut 

obliquely;  s,  intercalated  tubule;  s',  intercalated  tubule  r^ollc  anri  in  p^lictir  tnnirji 
in  cross  section  ;  b,  connective  tissue.  B,  cross  section  of  *-eUb  dllU  clli  Cld-bLlU  LUlucd. 
at)  excretory  duct  ;    e,  two-rowed  cylindrical  epithelium ;        tm-^t^i-i*o  T'T-.o-.t      otq      cut- 

b,  connective  tissue.  proprid.       J.  Hcy     are     bur- 

rounded  by  striated  muscle 
fibers,  chiefly  longitudinal.  The  lachrymal  sac,  which  is  provided  with 
small  branched  tubular  glands,  and  the  nasolachrymal  duct,  are  lined 
with  two-rowed  columnar  epithelium  and  a  lymphoid  tunica  propria,  which 
is  separated  from  the  underlying  periosteum  by  a  dense  plexus  of  veins. 


EAR. 

Development  and  General  Anatomy. 

The  ear  is  divided  into  three  parts:  the  external  ear,  including  the 
auricle  which  projects  from  the  surface  of  the  body,  and  the  external 
auditory  meatus  which  is  the  passage  leading  to  the  tympanic  membrane 
or  ''drum";  the  middle  ear,  including  the  tympanic  cavity  and  the  chain  of 
three  bones  extending  across  it;  and  the  internal  ear  which  is  a  system  of 
epithelial  ducts  in  connection  with  the  terminal  branches  of  the  acoustic 
nerve,  found  imbedded  in  the  temporal  bone. 

The  internal  ear  begins  as  a  local  thickening  of  the  epidermal  ecto- 
derm near  that  portion  of  the  medullary  tube  which  later  becomes  the  pons. 
The  thickened  areas  are  invaginated  as  shown  in  Fig.  429  A  and  B,  and 
the  pockets  thus  produced  become  separated  from  the  epidermis  in  the 
form  of  vesicles  [otocysts].     From  near  the  center  of  the  medial  surface 


DEVELOPMENT    OP   THE    EAR. 


379 


of  each,  an  ascending  tubular  outgrowth,  the  endolymphatic  duct,  arises, 
and  its  bhnd  termination  becomes  enlarged  to  form  the  endolymphatic 
sac.  The  duct  is  seen  in  section  in  Fig.  429,  C,  and  its  upper  end  projects 
above  the  rest  of  the  vesicle  in  Fig.  430,  A.  In  the  adult  it  terminates 
just  beneath  the  dura. 

In  two  places  the  medial  and  the  lateral  walls  of  the  upper  half  of  the 
vesicle  approach  one  another,  and  after  fusing  they  become  thin  and  rup- 
ture so  that  two  semicircular  ducts  are  formed  (Fig.  430,  B  and  C) .  The 
space  encircled  by  each  duct  may  be  regarded  as  a  hole  in  the  vesicle. 


Fig.  429. — Sections  of  Rabbit  Embryos  to  show  the  Development  of  the  Ear.  X  g- 
A,  9  days,  3.8  mm.;  B,  lo  days,  3.4  mm.;  C,  i2j^  days,  7.5  mm.;  D,  14  days,  10  mm.  a.,  Ectodermal  epithe- 
lium which  forms  the  membranous  internal  ear  ;  a.  bas.,  basilar  artery  ;  ch.  t.,  chorda  t^anpani ;  d.  C, 
cochlear  duct ;  d.  e.,  endolymphatic  duct ;  d.  s.  I.,  lateral  semicircular  duct ;  d.  S.  S.,  superior  semicir- 
cular duct;  ep.,  epidermis  ;  fa.,  facial  nerve;  meten.,  metencephalon  ;  m.  t.,  medullary  tube;  ph., 
pharynx. 


The  two  ducts  in  question  are  the  superior  and  posterior  semicircular  ducts 
respectively.  The  third  or  lateral  semicircular  duct  forms  soon  afterwards. 
In  Figs.  429  D  and  430  B  it  is  a  horizontal  shelf-hke  projection  of  the 
vesicle,  the  center  of  which  is  to  become  perforated  so  that  its  rim  forms  the 
duct.  The  portion  of  the  vesicle  which  receives  the  terminal  openings 
of  the  three  semicircular  ducts  is  called  the  utriculus.  Since  at  one  of 
their  ends  the  superior  and  posterior  ducts  unite  in  a  single  stalk  before 
entering  the  utriculus,  there  are  but  five  openings  for  the  three  ducts  (Fig. 
430  D).  Near  one  end  of  each  duct  there  is  a  dilatation  or  ampulla, 
where  nerves  terminate. 


38o 


HISTOLOGY. 


While  the  formation  of  the  semicircular  ducts  is  occurring  in  the  upper 
part  of  the  vesicle,  the  lower  portion  elongates  and  its  end  becomes  coiled, 
eventually  making  two  and  a  half  revolutions.  The  coiled  tube  is  the 
ductus  cochleae;  its  distal  end  is  the  caecum  cupularc  and  at  its 
proximal  end  is  the  caecum  vestibulare  (Fig.  430  D,  c.  v.).  A  dilated  sac 
formed  at  its  proximal  or  upper  end  opposite  the  caecum  vestibulare  is  the 
sacculus;  in  the  adult  the  connection  between  the  sacculus  and  ductus 
cochleae  is  relatively  narrow  and  is  called  the  ductus  reuniens  (Fig.  439). 
The  portion  of  the  original  vesicle  between  the  sacculus  and  utriculus, 
from  which  the  endolymphatic  duct  arises,  becomes  a  comparatively 
slender  tube,  the  ductus  utriculo-saccularis  (Fig.  439). 


Fig.  430. — Lateral  or  External  Siri-acks  of  Models   of  the    MEMBRANors   Portion   of   the 

Left  Lnternal  Ear  from  Hlman   Embryos.     Different  eTilar.tfenients.     (After  His,  Jr.) 
A.  from  an  emhr>o  of  6.9  mm.;   B,  10.2  mm.;  C.  13.5  mm.;  and   D,  22   mm.     am.,  ampulla;   c.  v.,   caecum 

vestibulare  of  d.  c,  cochlear  duct ;   d.  e.,  endolymphatic  duct ;   d.  S.  I.,  d.  S.  p.,   and    d.    S.   S.,   lateral, 

posterior,  and  superior  semicircular  ducts ;  sac.  sacculus  ;  ut..  utriculus. 

The  ectodermal  vesicle  thus  produces  a  complex  system  of  connected 
epithcHal  ducts,  which  are  the  superior,  posterior,  and  lateral  semicircular 
ducts,  the  utriculus,  the  utriculo-saccular  duct  with  the  endolymphatic 
duct  connected  with  it,  the  sacculus,  ductus  reuniens  and  ductus  cochleae. 
They  all  contain  a  fluid  called  endolymph.  The  acoustic  nerve  terminates 
in  branches  between  the  epithelial  cells  in  certain  parts  of  the  ducts. 
Round  areas  of  neuro-epithelium  are  called  maculae  acusticae;  there  is  one 
in  the  sacculus  and  another  in  the  utriculus.  Elongated  areas  are  cristac 
and  there  is  one  in  each  of  the  three  ampullae.  The  axis  or  modiolus, 
about  which  the  cochlear  duct  is  wound,  contains  the  nerves  which  send 
terminal  fibers  to  the  spiral  organ  of  the  adjoining  epithehum.  In  this 
they  form  a  line  of  terminations  along  the  medial  Avail  of  the  cochlear 
duct,  following  its  windings  from  base  to  cupula. 

The    mesenchyma   immediately    surrounding    the    system    of   ducts 


DEVELOPMENT    OF   THE   EAR. 


?8l 


becomes  mucoid  in  appearance  and  cavities  lined  with  mesenchymal 
epithelium  are  formed  within  it.  They  contain  a  tissue  fluid  called  peri- 
lymph. Around  the  semicircular  ducts  the  perilymph  spaces  are  so  large 
that  the  tissue  between  them  is  reduced  to  strands  as  sho^vn  in  Fig.  431; 
these  are  sometimes  called  ligaments.  The  perilymph  spaces  around  the 
semicircular  ducts  are  irregularly  arranged  and  communicate  with  one 
another  at  various  points,  but  those  around  the  cochlear  duct  form  a  single 
tube.  It  arises  from  the  other  spaces  at  the  base  of  the  cochlea  and  covers 
the  lateral  or  outer  surface  of  the  ductus  cochleae  as  it  ascends  to  the 


Semicircular  duct. 


Blood  vessel. 


Wall  of  the  semi- 
circular duct. 


Ligament. 


> f^t^^'],fy'^. Epi 


thelium  of  the 
duct. 


Perilymph  spaces. ^ 


Ligament  of  the 
duct. 


.   Bone  of  the  semi 
Ji'v      /      circular  canal. 


Blood  vessel. 

Fig.  431. — Cross  Section  of  a  Se-Micircui-ar  Duct  .\nd  the  Adjacent  Perilymph  Spaces  to- 
gether WITH  THE  Semicircular  C.an.JiL  of  Bone  in  which  they  are  Lodged.  Lrom  a  human 
adult.     X  50.     (Bohm  and  von  Davidoff.) 


cupula ;  there  it  turns  and  descends  along  the  medial  or  inner  surface  of  the 
ductus  cochleae,  ending  blindly  at  the  base  not  far  from  its  origin.  The 
ascending  perilymph  space  excavated  in  the  mesenchyma  around  the  coch- 
lear duct  is  the  5ca/a  vestihuli.  The  descending  space  with  which  it  con- 
nects at  the  cupula  is  the  scala  tympani.  The  arrangement  of  the  cochlear 
duct  and  its  scalae  is  shown  in  the  section  through  the  axis  of  the  spiral, 
Fig.  432.  The  upper  side  of  the  figure  is  directed  forward  and  outward 
in  relation  to  the  body. 

The  temporal  bone  develops  from  the  mesenchyma  surrounding  the 


382 


HISTOLOGY, 


ducts  and  their  perilymph  spaces,  so  that  when  the  membranous  labyrinth 
which  they  form  is  removed  by  maceration,  the  bone  still  contains  a 
corresponding  arrangement  of  cavities  and  canals.  These  constitute  the 
bony  labyrinth.  Casts  of  it  made  in  soft  metal  may  be  seen  in  all  anatom- 
ical museums.  Instead  of  subdivisions  to  correspond  with  the  utriculus, 
sacculus,  and  utriculo-saccular  duct,  the  bony  labyrinth  has  a  single  space 
called  the  vestibule.     Into  it  the  semicircular  and  cochlear  canals  open. 

The  middle  and  the  external  ear  arise  in  connection  with  the  first  or 
spiracular  gill  cleft.  In  common  with  the  other  clefts  this  includes  an 
entodermal  pharyngeal  outpocketing  (Fig.  i88,  p.  i66)  and  an  ectodermal 
depression  (Fig.  187,  sp.),  which  meet  one  another.  The  latter  becomes 
surrounded  bv  several  nodular  elevations  which  coalesce  in  a  definite 


Modiolus 


Ganglion 
spirale. 


Scala  vestibuli. 


'   Scala  tynipani. 


Ramus 
vestibularis 


of  the 

nervus 

acusticus. 


Meatus  acusticus  internus. 

Fig.  432.— Horizontal  Section  of  the  Cochlea  of  a  Kitten,    x  8. 

The  winding  ductus  cochlearis,  x,  crossed  the  plane  of  section  five  times.     Above  it  in  every  case  is  the 

sca/a  vestibuli,  and  below  it  is  the  scala  tympatii. 

manner  to  make  the  projecting  auricle  [pinna]  of  the  external  ear.  Its 
depression  deepens,  becoming  the  external  auditory  meatus,  the  ectoderm 
at  the  bottom  of  which  passes  over  the  tympanic  membrane,  thus  forming 
its  outer  layer.  The  entodermal  portion  of  the  spiracular  cleft  becomes  in 
the  adult  an  elongated  outpocketing  of  the  pharynx,  known  as  the  auditory 
tube  [Eustachian  tube].  As  seen  in  the  section  Fig.  433,  the  tube  is  sepa- 
rated from  the  bottom  of  the  meatus  by  a  very  thin  layer  of  mesenchyma. 
In  the  mesenchyma  behind  the  spiracular  cleft  a  chain  of  three  small 
bones  (the  malleus,  incus,  and  stapes)  develops ;  it  extends  from  the  bottom 
of  the  meatus  to  the  vestibule.  The  bony  wall  of  the  vestibule  is  deficient 
at  the  small  oval  area  where  the  stapes  reaches  it,  so  that  the  chain  of  bones 
comes  directly  in  contact  with  the  fibrous  covering  of  the  perilymph  space. 


DEVELOPMENT   OF   THE    EAR.  383 

This  area  of  contact  is  the  jenestra  vestibuli  (fenestra  meaning  window). 
When  the  chain  of  bones  vibrates  back  and  forth,  the  motion  of  the  stapes 
is  transmitted  through  the  fenestra  vestibuH  to  the  perilymph,  and  waves 
may  pass  up  the  scala  vestibuH  and  down  the  scala  tympani,  stimulating 
the  nerves  of  hearing  in  the  cochlear  duct.  The  blind  termination  of 
the  scala  tympani  rests  against  the  lateral  wall  of  the  vestibule  where  also 
the  bone  fails  to  develop;  the  round  jenestra  cochleae  is  thus  produced. 
Its  fibrous  membrane  may  yield  somewhat  to  the  perilymph  waves,  thus 
relieving  tension  in  the  cochlea. 

1  s.tr.  ''^:Wm-mP-^\:-  A^V- 


d.c.' 


Fig.  433. — Horizontal  Section  through  the  Ear  of  a  Human  Embryo  of  about  5  Cms. 
au.,  Auricle;  au.t.,  auditory  tube;  ch.t.,  chorda  tympani;    d.C,  cochlear  duct;   d.S.I.  and  d.S.p.,  lateral 
and  posterior  semicircular  ducts;  e.a.m.,  external  auditory  meatus;  fa.,  facial  nerve;  f.c,  fenestra 
cochleae;  p. s.,  perilymphatic  space  ;  St..  stapes  ;  s.tr.,  transverse  sinus  ;  t.b.,  temporal  bone. 

In  Fig.  433  the  fragments  of  the  chain  of  bones  together  with  neigh- 
boring nerves  are  imbedded  in  a  mass  of  mesenchyma.  In  a  later  stage 
the  outer  end  of  the  auditory  tube  expands,  filling  all  the  space  between 
the  vestibule  and  the  bottom  of  the  meatus.  Thus  it  forms  the  tympanic 
cavity.  It  encounters  the  chain  of  bones  and  the  chorda  tympani,  and 
wraps  itself  around  them  so  that  they  lie  in  its  folds  or  plicae.  Thus  all 
structures  which  extend  into  the  tympanic  cavity,  or  appear  to  cross  it, 
are  covered  with  a  layer  of  entodermal  epithehum  derived  from  the  audi- 
tory tube.  The  original  contact  between  the  ectoderm  and  entoderm 
of  the  spiracular  cleft  forms  only  an  insignificant  part  of  the  tympanic 
membrane.  The  latter  becomes  greatly  enlarged,  extending  somewhat 
along  the  upper  surface  of  the  ectodermal  auditory  meatus.     The  portion 


384  HISTOLOGY. 

of  the  malleus  lying  near  it  becomes  imbedded  in  its  mesenchymal  layer, 
and  its  inner  entodermal  layer  is  made  by  the  expansion  of  the  tympanic 
cavity.  The  enlargement  of  the  tympanic  cavity  continues  after  birth 
when  it  invades  the  spaces  formed  within  the  mastoid  part  of  the  temporal 
bone. 

In  spite  of  these  modifications  the  course  of  the  spiracular  cleft  is 
retained  in  the  adult.  The  ectodermal  depression  and  its  surrounding 
elevations  constitute  the  external  ear;  the  pharyngeal  outpocketing  persists 
as  the  auditory  tube  and  the  tympanic  cavity  of  the  middle  ear.  It  opens 
freely  into  the  pharynx  and  contains  air. 

Sacculus,  Utriculus,  and  Semicircular  Ducts. 
The  walls  of  all  these  structures  consist  of  three  layers.     On  the  out- 
side there  is  connective  tissue  with  many  elastic  fibers  and  occasional  pig- 
ment cells.     This  is  followed    by  a   narrow  basement  membrane  said  to 
form  small  nodular  elevations   toward   the   third   and   innermost   layer, 
the  simple  flat  epithelium.      Near  the  maculae  and  cristae  the  connective 
tissue  and  the  basement  membrane  become  thicker;    the  epithehal  cells 
are  columnar  with  a  cuticular  border.     In  the  neuro-epithelium  of  these 
areas   there   are   two   sorts  of  cells,  sustentacular  and  hair  cells.     The 
sustentacula}'  or  fiber  cells  extend  clear  across  the  epithelium  and  are  some- 
what expanded  at  both  ends;  they  contain  oval  nuclei.     Hair  cells,  which 
receive  the  stimuli,  are  columnar  cells  limited  to  the 
(£?  superficial    half    of    the  epithelium;    they  have  large 

"^Q  spherical  nuclei  near  their  rounded  basal  ends,  and  a 

^^  ^  '^  clump  of  fine  agglutinated  filaments  projecting  from 

'^        ^  their  free  surface.     The  nerves  lose   their   myehn   as 

"^   ^  they  enter  the  epithchum  and  ascend  to  the  bases  of 

Fig.  434. —  Otoconia      the  hair  cclls.     There  they  bend  laterally,  forming  a 

FROM     THE    SACCI-  •'  . 

Lis^F  AN  iNKANT.  (Jensc  nctwork  which  appears  as  a  granular  layer  in  or- 
dinary preparations;  the  granules  are  optical  sections 
and  varicosities.  The  horizontal  fibers  terminate  like  their  occasional 
branches,  by  ascending  between  the  hair  cells,  on  the  sides  of  which  they 
form  pointed  free  endings.  They  do  not  reach  the  free  surface  of  the 
epithelium.  This  surface  is  covered  by  a  continuation  of  the  cuticula, 
a  "membrana  hmitans,"  which  is  perforated  by  the  hairs.  Over  the  two 
maculae  there  is  a  soft  substance  containing  very  many  crystals  of  calcium 
carbonate,  1-15  y.  long,  which  are  named  otoconia.  (Large  "ear  stones" 
of  fishes  are  called  otoHths.)  Over  the  cristae  of  the  semicircular  ducts' 
there  is  a  gelatinous  substance,  transparent  in  fresh  preparations,  but 
coagulated  and  rendered  visible  by  reagents. 


COCHLEA. 


385 


The  "Kgaments"  of  the  ducts,  the  thin  periosteum  of  the  bony  semi- 
circular canals,  and  the  perilymph  spaces  lined  with  mesenchymal  epithe- 
lium are  seen  in  Fig.  431. 

Cochlea. 

The  relation  between  the  ductus  cochleae  and  the  scalae  t}Tiipani  and 
vestibuli  is  shown  in  Fig.  435.  The  ductus  is  triangular  in  cross  section, 
being  bounded  on  its  peripheral  surface  by  the  thick  periosteum  of  the 
bony  wall  of  the  cochlea;  on  its  apical  surface  (toward  the  cupula)  by  the 
membrana  vestibularis  [Reissner's  membrane];  and  on  its  basal  or  medial 
surface  by  the  lamina  spiralis.     These  three  walls  may  be  described  in  turn. 

The  peripheral  wall  of  the  cochlear  duct  is  formed  by  the  dense  fibrous 
periosteum  attached  to  the  bone,  together  with  a  large  mass  of  looser  tissue 


Blood  vessels 


Limbus  Membrana 

spiralis.  vestibularis. 


Scala  vestibuli. 


Ductus 
,"'  cochlearis. 


;-r— ""^T  \'as  prominens. 


"^  Ligamentum 
spirale. 


Ganglion  spirale.  /% 

I  . 
I 

Scala  tympani. 


Lamina  spiralis  ossea.  Lamina  spiralis  membranacea. 

Fig.  435. — The  Portion  of  Figure  432  marked  "Scala  vestibuli"  and  "Scala  tympani."     X  50. 

crescentic  in  cross  section,  the  ligamentmn  spirale  (Fig.  435).  The  spiral 
ligament  is  covered  by  a  layer  of  cuboidal  epitheHal  cells  belonging  to  the 
cochlear  duct.  Close  beneath  the  epithehum  there  are  blood  vessels  which 
are  said  to  give  rise  to  the  endolymph.  The  thick  plexus  which  they  form 
is  described  as  a  band,  the  stria  vascularis,  which  terminates  more  or  less 
distinctly  with  the  vas  prominens.  The  latter  occupies  a  low  elevation 
of  tissue  which  has  its  maximum  development  in  the  basal  coil  of  the 
cochlea  (Fig.  435). 


386 


HISTOLOGY. 


The  apical  wall  or  membrana  vestibularis  consists  of  a  thin  layer  of 
connective  tissue  bounded  on  one  side  by  the  mesenchymal  epitheHum 
of  the  scala  vestibuh,  and  on  the  other  by  the  simple  flattened  ectodermal 
epithelium  of  the  cochlear  duct. 

The  basal  wall  or  lamina  spiralis  extends  from  the  modiolus  per- 
ipherally to  the  bony  wall  of  the  cochlea.  Near  the  modiolus  it  lies  between 
the  two  scalae  but  peripherally  it  is  between  the  ductus  cochleae  and  the 
scala  tympani.  Toward  the  modiolus  it  contains  a  plate  of  bone  perforated 
for  the  passage  of  vessels  and  nerves;  this  part  is  the  lamina  spiralis  ossea. 
The  peripheral  portion  is  the  lamina  spiralis  membranacea.*     Both  parts 


Membrana  tectoria. 

i 


Hair  cells. 


Labium  vestibulare.        Sulcus  spiralis. 
/ 
/ 


Capillaries  of  tlie  stria. 


Hensen's  Claudius's        ^  ,-^ 

cells.  iiJ*\>.  - 

"^  ^  1* 


Nerve  bundle. 


Labium        Inner  Outer 

tympanicum.~^       ■■* — -~^^n.»-^-- 
Pillar  cells. 


Deiters's    Membrana     Connective 
cells.         basilaris.  tissue. 


Fk;.  436.  — Portion  of  Figurk  435.     X  240.     x.  Intercellular  "  tunnel  "  traversed  by  nerve  fibers. 


are  covered  below  by  the  mesenchymal  epithelium  of  the  scala  tympani, 
and  above  by  the  epithelium  of  the  cochlear  duct  including  its  complex 
neuro-epithelium  known  as  the  spiral  organ  [of  Corti]. 

Where  the  membrana  vestibuli  meets  the  osseous  spiral  lamina  there  is 
an  elevation  of  tough  connective  tissue  called  the  limhus  spiralis  (Fig.  435). 
It  consists  of  abundant  spindle-shaped  cells  and  blends  below  with  the 
periosteum  of  the  spiral  lamina.     Superficially  it  produces  irregularly  hemi- 


*  The  familiar  term  lamina  spiralis  memhranacea  employed  by  Professor  Stohr  is  not 
included  among  the  Nomina  Anatomica.  In  place  of  it  is  lamina  basilaris.  Whether  the 
latter  should  be  considered  synonymous  with  the  former,  or  should  refer  fo  the  entire  basal 
layer  into  a  portion  of  which  a  lamina  spiralis  ossea  projects,  is  not  apparent. 


COCHLEA.  387 

spherical  papillae  found  within  the  cochlear  duct  near  the  vestibular 
membrane.  Further  within  the  ductus  cochleae  there  is  a  row  of  flat 
elongated  forms  directed  from  the  modiolus  toward  the  periphery;  these 
are  sometimes  called  Huschke's  auditory  teeth  (Fig.  438).  The  papillae 
are  covered  by  a  simple  layer  of  fiat  epithelium.  As  the  limbus  extends 
from  the  vestibiilar  membrane  toward  the  peripheral  part  of  the  cochlea, 
it  terminates  abruptly  in  an  overhanging  labium  vestibulare  beneath  which 
is  an  excavation,  the  sulcus  spiralis  (Fig.  436) .  The  basal  wall  of  the 
sulcus  is  the  labium  tympanicum,  found  at  the  peripheral  edge  of  the 
osseous  spiral  lamina.  As  the  epithelium  of  the  limbus  passes  over  the 
labium  vestibulare  into  the  sulcus,  it  becomes  cuboidal.  A  remarkable 
formation,  non-nucleated,  soft  and  elastic,  projects  from  the  labium  ves- 
tibulare over  the  neuro-epithelium  of  the  membra- 
nous spiral  lamina.  It  is  called  the  membrana  tectoria 
and  is  considered  to  be  a  cuticular  formation  of  the 
labial  cells  to  which  it  is  attached. 

The  lamina  spiralis  membranacea,  or  lamina  basil- 
aris   (?),   consists  of  four  layers.      The  mesenchymal 
epithelium  of  the  scala  tympani  is  followed  by   a  layer 
of  delicate  connective  tissue  prolonged  from  the  peri-      p^^,  4,7 —surface 
osteum  of   the    scala.      Its  spindle  cells   are    at   right  lamina  °sviIS.^ 

angles   with    the   fibers    of    the  overlying    membrana  i  ^clrf '^■^^x'^240^ 

basilaris.     This    membrane,    wdiich    is    beneath    the  offo^us^"    ^  ^"^^ 

epithelium   of  the   cochlear   duct,    consists   of   coarse      ®'  ^ciaud/us")'  ^of  \he 
straight  fibers  extending  from  the  labium  tympanicum  inTocus ;  '?°fibersof 

...  .       ,  .  .  the  membrana  basil- 

to  the  hgamentum  spirale.      ihey  cause   it  to  appear  aris  in  focus;    b, 

nuclei  of  the  under- 

fkiely   striated   (Fig.  437).     Peripherally   (beyond  the  lyingconnectivetis- 

bases  of  the  outer  pillar  cells)    the   fibers  are   thicker 
and  are  called  "  auditory  strings " ;  they  are  shortest   in   the   basal  part 
of  the  cochlea  and  longest  toward  the  apex,  corresponding  in  length  with 
the  basal  layer  of  the  cochlear  duct.      These  fibers  have  been   thought 
to  vibrate  and  assist  in  conveying  sound  waves  to  the  nerves. 

The  epithehal  cells  covering  the  basilar  layer,  present  rows  of  higlily 
modified  forms  extending  up  and  down  or  lengthwise  of  the  cochlear  duct, 
and  constituting  the  spiral  organ  [of  Corti].  Next  to  the  cuboidal  epithehum 
of  the  sulcus  spiralis  there  is  a  single  row  of  inner  hair  cells  (Fig.  436). 
These  are  short  columnar  cells  which  do  not  reach  the  bottom  of  the  epi- 
thelium; each  has  about  forty  long  stifi^  hairs  on  its  free  surface.  The 
inner  hair  cells  are  followed  peripherally  by  two  rows  of  pillar  cells,  the 
inner  and  outer,  which  extend  the  whole  length  of  the  cochlear  duct.  As 
seen  in  cross  section  they  are  in  contact  above,  but  are  separated  below 


388 


HISTOLOGY. 


by  a  triangular  intercellular  space  or  "tunnel."  The  space  is  filled  with 
soft  intercellular  substance.  Thus  they  rest  upon  the  basilar  membrane 
in  A-form.  The  inner  pillar  cells  are  said  to  be  more  numerous  than  the 
outer.  Both  forms  are  stiff  bands  with  triangular  expanded  bases,  which 
are  associated  with  nucleated  masses  of  protoplasm  within  the  "tunnel." 
The  "heads"  or  upper  ends  interlock,  since  the  inner  pillars  are  concave 
to  receive  the  convex  surface  of  the  outer  pillars.  From  the  superficial 
surface  of  both,  plates  extend  peripherally  or  outward,  that  of  the  inner 
pillar  partly  covering  the  head-plate  of  the  outer  pillar  (Fig.  438).     The 


Xerve 


'-^     First       j      '  Tun-    1        W 
spiral  nel.      j 

cord.       Vasspirale 
Inner  Outer 


Pillar  cells. 


Fig.  438. — Di.AGRAM  of  the  Stri^cture  of  the  Basal  W.\ll  of  the  Duct  of  the  Cochlea. 
A,  View  from  the  side.     B,  View  from  the  surface.     In  the  latter  the  free  surface  is  in  focus.     It  is  evident 
that  the  epithelium  of  the  sulcus  spiralis,  lying  in  another  plane,  as  well  as  the  cells  of  Claudius,  can 
be  seen  distinctly  only  by  lowering   the  tube.     The  mcmbrana   tecloria   is  not  drawn,     'i'he  spiral 
nerve  bundles  are  indicated  bv  dots. 


dark  bodies  in  the  heads  of  both  pillars,  and  in  the  basal  part  of  the  outer 
ones,  are  not  nuclei. 

On  the  peripheral  side  of  the  outer  pillars  there  are  several  rows 
(usually  four)  of  outer  hair  cells  separated  from  one  another  by  sustentac- 
ular  cells  f  Deitcrs's  cells).  The  outer  hair  cells  have  shorter  hairs  than  the 
inner  ones,  which  otherwise  they  resemble.  They  do  not  extend  to  the 
basilar  membrane,  thus  leaving  unoccupied  the  communicating  inter- 
cellular (Nuel's)  spaces  between  the  deeper  portions  of  the  sustentacular 
cells.     Nuel's  spaces  connect  with  the  tunnel.     The  sustentacular  cells 


COCHLEA.  •  389 

are  slender  bodies  each  containing  a  stiff  filament.  They  have  a  cap- 
like cuticular  border  so  that  they  remotely  resemble  the  distal  phalanges 
of  the  fingers.  The  spaces  between  the  "phalanges"  are  occupied  by  the 
outer  hair  cells.  The  cuticular  expansions  connect  with  one  another 
forming  a  reticular  membrane,  into  the  apertures  of  which  the  hair  cell 
processes  extend.  The  sustentacular  cells  resemble  the  pillar  cells,  but 
their  transformation  into  stiff  fibers  has  not  proceeded  so  far;  the  cutic- 
ular border  is  comparable  with  the  head  plate.  The  most  peripheral  of 
the  sustentacular  cells  are  followed  by  elongated  columnar  cells  (cells  of 
Hansen)  which  gradually  shorten  and  pass  into  the  undifferentiated  epi- 
thehum  of  the  cochlear  duct.  The  low  cells  following  Hensen's  cells  are 
the  cells  of  Claudius.  They  are  said  to  have  branching  bases  which  extend 
deep  into  the  underlying  tissue.  In  both  the  columnar  and  the  low  forms 
there  are  single  stiff  filaments  which  are  less  developed  than  in  the  susten- 
tacular cells.     The  centrosomes  of  all  these  cells  He  near  their  free  surfaces. 

Nerves  and  Vessels  of  the  Labyejxth. 

The  acoustic  nerve  is  a  purely  sensor}^  nerve  passing  between  the  pons 
and  internal  ear  through  a  bony  canal,  the  internal  auditory  meatus.  It  is 
divided  into  vestibular  and  cochlear  portions  (Fig.  432).  The  vestibular 
nerve  proceeds  from  the  vestibular  ganglion  and  has  four  branches;  the 
utricular  nerve  and  the  superior,  lateral,  and  posterior  ampullary  nerves. 
Their  terminations  have  already  been  described  (p.  384).  The  cochlear 
nerve,  which  has  a  saccular  branch,  proceeds  from  the  spiral  ganglion 
lodged  within  the  modiolus  at  the  root  of  the  lamina  spirahs  (Figs.  432  and 
435).  The  ganghon  cells  remaui  bipolar  hke  those  of  embryonic  spinal 
ganglia.  The  neuraxon  and  the  suigle  peripheral  dendrite  are  medullated 
except  near  the  cell  body.  The  peripheral  fibers  extend  through  the  lamina 
spirahs  ossea,  within  which  they  form  a  wide  meshed  plexus,  and  after 
losing  their  myelin  they  emerge  from  its  free  border  through  the  foramina 
nervosa.  In  continuing  to  the  spiral  organ  they  curve  in  the  direction  of 
the  cochlear  windings,  thus  producing  spiral  strands.  Those  nearest  the 
modiolus  are  on  the  axial  side  of  the  pillar  cells;  the  middle  ones  are  between 
the  pillars,  in  the  tunnel;  and  the  outer  ones  are  beyond  the  pillar  cells. 
From  these  bundles  dehcate  fibers  pass  to  the  hair  cells,  on  the  sides  of 
which  they  terminate. 

The  internal  auditory  artery  is  a  branch  of  the  basilar  artery.  It 
arises  in  connection  with  branches  which  are  distributed  to  the  under 
side  of  the  cerebellum  and  the  neighboring  cerebral  nerves,  and  passes 
through  the  internal  auditory  meatus  to  the  ear.  It  divides  into  vestibular 
and  cochlear  branches.     The  vestibular  artery  supphes  the  vestibular  nerve 


390 


HISTOLOGY. 


and  the  upper  lateral  portion  of  the  sacculus,  utriculus  and  semicircular 
ducts.  The  cochlear  artery  sends  a  vestibulo-cochlear  branch  to  the  lower 
and  medial  portion  of  the  sacculus,  utriculus,  and  ducts.  This  branch 
also  supplies  the  first  third  of  the  first  turn  of  the  cochlea.  The  capillaries 
formed  by  the  vestibular  branches  are  generally  wide  meshed,  but  near  the 
maculae  and  cristae  the  meshes  are  narrower.     The  terminal  portion  of 


Arteria  audiliva  J 
iiilenia 


Arteria  vestibularis 
Arteria  cochlearis.  _ 


Ductus  semicircularis 
superior. 

Ampulla  lateralis. 


\'ei)a  aquaeductus 
vestibuli. 
:  '   ''       Ductus  semicircularis 
/  lateralis. 


Arteria 
cochlearis. 

Vestibulo-cochlear 
branch     of     the           \'eiia  spinalis.                   \'ena  vestibularis, 
arteria  coclilearis.  > , < 


Ductus  semicircularis 
posterior. 


\'ena  aquaeductus  cochleae. 

Fig.  439.— Di.\gram  of  the  Blood  Vessels  of  the  Right  Hlm.\n  Labyrinth.    Medial  and  Pos- 
terior Aspect. 
D.C.  Ductus  cochlearis:  S.,  sacculus  ;    U.,  utriculus  ;    i,  ductus  reutiieus;    2,  ductus  utriculo-saccularis. 
The  saccus  endolymphaticus  is  cut  off. 

the  cochlear  artery  enters  the  modiolus  and  forms  three  or  four  spirally 
ascending  branches.  These  give  rise  to  about  thirty  radial  branches 
distributed  to  three  sets  of  capillaries  (Fig.  440);  i,  those  to  the  spiral 
ganglion;  2,  those  to  the  lamina  spiralis;  and  3,  those  to  the  outer  walls 
of  the  scalae  and  the  stria  vascularis  of  the  cochlear  duct. 

The  veins  of  the  labyrinth  form  three  groups,     i.  The  vena  aquae- 


BLOOD    VESSELS    OF    THE    INTERNAL    EAR. 


391 


ductus  vestihuli  receives  blood  from  the  semicircular  ducts  and  a  part  of  the 
utriculus.  It  passes  toward  the  brain  in  a  bony  canal  along  with  the  ductus 
endolymphaticus,  and  empties  into  the  superior  petrosal  sinus.  2.  The 
vena  aquaeductus  cochleae  receives  blood  from  parts  of  the  utriculus,  saccu- 
lus  and  cochlea;  it  passes  through  a  bony  canal  to  the  internal  jugular  vein. 
Within  the  cochlea  it  arises,  as  shown  in  Fig.  440,  from  small  vessels 
including  the  vas  prominens  (a)  and  the  vas  spirale  (6).  Branches  derived 
from  these  veins  pass  toward  the  modiolus.     (There  are  no  vessels  in  the 

Scala  tympaiii.     Scala  vestibuli. 


Stria  ^asculan 


/  Cross  section  of  a  spiral 
/      artery  of  the  modiolus. 


Vena  laminae  spiralis. 


Gang-lion  spirale. 


--  Vena  spiralis  superior. 


Cross  section  of  a  spiral 
artery  of  the  modiolus 


Vena  laminae  spiralis. 


Anastomosis. 


"~*Vena  spiralis  inferior. 


Fig.  440. — Diagram  of   a  Section  of  the   First  (Basal)  and  Second  Turns  of  the  Cochlea. 
a,  Vas  prominens  ;      b,   vas  spirale. 

vestibular  membrane  of  the  adult,  and  the  vessels  in  the  wall  of  the  scala 
tympani  are  so  arranged  that  only  veins  occur  in  the  part  toward  the  mem- 
branous spiral  lamina;  thus  the  latter  is  not  affected  by  arterial  pulsation.) 
Within  the  modiolus  the  veins  unite  in  an  inferior  spiral  vein  which  receives 
blood  from  the  basal  and  a  part  of  the  second  turn  of  the  cochlea,  and  a 
superior  spiral  vein  which  proceeds  from  the  apical  portion.  These  two 
spiral  veins  unite  with  vestibular  branches  to  form  the  vena  aquaeductus 
cochleae    (Fig.   439).     3.  The   internal  auditory   vein   arises   within   the 


392 


HISTOLOGY. 


modiolus  from  the  veins  of  the  spiral  lamina;  these  anastomose  with  the 
spiral  veins  (Fig.  440).  It  receives  branches  also  from  the  acoustic  nerve 
and  from  the  bones,  and  empties  "in  all  probabiHty,  into  the  vena  spinaHs 
anterior."  (The  transverse  and  petrosal  sinuses  are  often  said  to  receive 
this  vein ;  and  the  vena  aquaeductus  vestibuh  has  been  described  as  entering 
the  inferior  instead  of  the  superior  petrosal  sinus.) 

Lymphatic  spaces  within  the  internal  ear  are  represented  by  the 
perilymph  spaces  which  communicate  through  the  aquaeductus  cochleae 
with  the  subarachnoid  space;  the  connecting  structure  or  "ductus  peri- 
lymphaticus"  is  described  as  a  lymphatic  vessel.  The  saccus  endolymph- 
aticus  which  is  the  dilated  distal  end  of  the  endolymphatic  duct,  is  in  con- 
tact with  the  dura,  and  there  are  said  to  be  openings  between  it  and  the 
subdural  space.  In  the  internal  ear  perivascular  and  perineural  spaces 
are  found,  and  they  probably  connect  with  the  subarachnoid  space. 


..'#?. 


Cartilage. 


^i,''^''.  ^1^' 


Mucosa  of  the 

phai  ynx. 


Glands. 


Fig.  44r.— Cross  Smctio.v  ok   ihh  (ak  i  u  ac.inocs  Part  of  the  Auditory  Ti-bk. 
(Eolini  and  von  DavidotT.) 


X  12. 


Middle  Ear. 
The  tympanic  cavity,  which  contains  air,  is  lined  with  a  mucous 
membrane  closely  connected  with  the  surrounding  periosteum.  It  consists 
of  a  thin  layer  of  connective  tissue  covered  generally  with  a  simple  cuboidal 
epithehum.  In  places  the  epithehal  cells  may  be  flat,  or  tall  with  nuclei 
in  two  rows.     Cilia  are  sometimes  widely  distributed  and  are  usually  to  be 


MIDDLE    EAR.  393 

found  on  the  floor  of  the  cavity.  In  its  anterior  part,  small  alveolar  mu- 
cous glands  occur  very  sparingly.  Capillaries  form  wide  meshed  networks 
in  the  connective  tissue,  and  lymphatic  vessels  are  found  in  the  periosteum. 
The  auditory  tube  includes  an  osseous  part  toward  the  tympanum,  and 
a  cartilaginous  fart  toward  the  pharynx.  Its  mucosa  consists  of  fibrillar 
connective  tissue,  together  with  a  ciliated  columnar  epithelium  which  be- 
comes stratified  as  it  approaches  the  pharynx.  The  stroke  of  the  cilia  is 
toward  the  pharyngeal  orifice.  In  the  osseous  portion  the  mucosa  is 
without  glands  and  very  thin;  it  adheres  closely  to  the  surrounding  bone. 
Along  its  floor  there  are  pockets  containing  air,  the  ceUulae  pneu- 
maticae.  In  the  cartilaginous  part  the  mucosa  is  thicker;  near  the  pharynx 
it  contains  many  mucous  glands  (Fig.  441).  Lymphocytes  are  abundant 
in  the  surrounding  connective  tissue,  forming  nodules  near  the  end  of  the 
tube  and  blending  with  the  pharyngeal  tonsil.  The  cartilage,  which  only 
partly  surrounds  the  auditory  tube,  is  hyaline  near  its  junction  with  the 
bone  of  the  osseous  portion;  it  may  contain  here  and  there  coarse  fibers 
which  are  not  elastic.  Distally  the  matrix  contains  thick  nets  of  elastic 
tissue,  and  the  cartilage  is  consequently  elastic. 

External  Ear. 

Between  the  middle  ear  and  the  external  ear  is  the  tympanic  membrane, 
which  consists,  from  without  inward,  of  the  following  strata:  the  cutaneum, 
radiatum,  circular e  and  mucosum  (Fig.  442). 
The  stratum  cutaneum  is  a  thin  skin  without 
papillae  in  its  corium,  except  along  the  handle 
or  manubrium  of  the  malleus.  There  it  is  a 
thicker  layer  containing  the  vessels  and  nerves 
which  descend  along  the  manubrium  and  spread 
from  it  radially.  In  addition  to  the  venous 
plexus   which    accompanies    the    artery    in    that 

• ,  •  ,^  •  ^  r         •  1     l^  F1G.442. — Cross  Section  OF 

Situation,  there  is  a  plexus  oi  veins  at  the  per-  the   membrana  tym- 

.  .  PANI    BELOW   THE    MANU- 

iphery   of   the    membrane.     The   latter  receives  brium.   x   450.    (After 

^  -'  Kolliker.) 

vessels   both  from  the    stratum    cutaneum    and  .     a,  stratum  cutaneum  (show- 

ing     the     corneum    and 

the  less  vascular  stratum  mucosum.     The  radiate  germinativum);  b,  strat- 

umradiatuni,     its      fibers 

and  circular  strata  consist  of  compact  bundles  of  cut  across ;  c,  stratum  cir- 

^  culare ;     d,   stratum  mu- 

fibrous  and  elastic  tissue  arranged  so  as  to  sug-  cosum. 

gest  tendon.     The  fibers  of  the  radial  layer  blend 

with  the  perichondrium  of  the  hyaline  cartilage  covering  the  manubrium. 
Peripherally  the  fiber  layers  form  a  fibro-cartilaginous  ring  which  con- 
nects with  the  surrounding  .bone.  The  stratum  mucosum  is  a  thin  layer 
of  connective  tissue  covered  with  a  simple  non-cihated   flat  epithehum 


594 


HISTOLOGY. 


continuous  with  the  hning  of  the  tympanic  cavity.  Peripherally,  in 
children,  its  cells  may  be  taller  and  ciliated.  As  a  whole  the  tympanic 
membrane  is  divided  into  tense  and  faccid  portions.  The  latter  is  a  rela- 
tively small  upper  part  in  which  the  fibrous  layers  are  deficient. 

The  external  auditory  meatus  is  lined  with  skin  continuous  with  the 
cutaneous  layer  of  the  tympanic  membrane.  In  the  deep  or  osseous  portion 
the  skin  is  very  thin,  without  hair  or  glands  except  along  its  upper  wall. 
There  and  in  the  outer  or  cartilaginous  part  ceruminons  glands  are  abun- 
dant. "They  are  branched  tubulo-alveolar  glands"  (Huber)  which  in 
many  respects  resemble  large  sweat  glands.     Their  ducts  are  lined  with 

Epidermis.    //  v 


Hair  sheath. 
Corium. 


Excretory  duct. 


Membrana  propria. 

Nuclei  of  smooth  muscle  fibers. 

Secretion. 

Gland  cells. 


Young  hair.  — '^-- 


Fig.  443.— From  a  \'ertic.ai.  Section  through 

THE      SKI.V      of     the      ExTERN.-VL      AUDITORY 

Me.atus  of   ax   Infant,     x  50.    The  excre- 
tory duct  opens  into  the  hair  foflicle. 


Secretion. 

,—  Cuticular  border. 
^^^^_^^^=^=/_  Gland  cells. 
'  C\'^  c\'~rfcA^.       Nuclei  of  smooth  muscle 
-=i=»;^^,.£-=S=?  fibers. 

Membrana  propria. 


Fig.  444. — Tubules  of  the  Ceruminous  Glands. 
A,  Cross  section,  from  an  infant;  B,  longitu- 
dinal rection,  from  a  boy  12  years  old. 


stratified  epithehum.  The  coils  consist  of  a  single  layer  of  secreting  cells, 
general  cuboidal,  surrounded  by  smooth  muscle  fibers  and  a  wtII  defined 
basement  membrane.  They  differ  from  sweat  glands  in  that  their  coils 
have  a  very  large  lumen  especially  in  the  adult,  and  their  gland  cells, 
often  with  a  distinct  cuticular  border,  contain  many  pigment  granules  and 
fat  droplets.  Their  narrow  ducts  in  adults  end  on  the  surface  of  the  skin 
close  beside  the  hair  sheaths;  in  children  they  empty  into  the  sheaths  (Fig. 
443).  The  secretion  consists  of  pigment,  fat,  and  fatty  cells,  the  latter  de- 
rived probably  from  the  hair  sheaths. 

The  cartilage  of  the  external  auditory  meatus  and  of  the  auricle  is 
elastic. 


THE    NASAL    CAVITIES. 


395 


NOSE. 

The  nasal  cavities  are  formed  by  the  invagination  of  a  pair  of  epi- 
dermal thickenings  similar  to  those  which  give  rise  to  the  lens  and  auditory 
vesicle.  The  pockets  thus  produced  in  the  embryo  are  called  "nasal  pits" 
(Fig.  187,  n,  p.  166).  Their  external  openings  remain  as  the  nares  of  the 
adult.  Temporarily,  from  the  third  to  the  fifth  month  of  fetal  Hfe,  they  are 
closed  by  an  epitheHal  proliferation.  Each  nasal  pit  acquires  an  internal 
opening,  choana,  in  the  roof  of  the  pharynx.  The  choanae  are  at  first  situ- 
ated near  the  front  of  the  mouth,  separated  from  one  another  by  a  broad 
nasal  septum  (Fig.  445).  As  the  latter  extends  posteriorly  it  is  joined  by 
the  palate  processes  which  grow  toward  it  from  the  sides  of  the  maxillae. 
Thus  the  choanae  recede  toward  the  back  of  the  mouth  while  the  em- 
bryonic condition  of  cleft  palate  is  being  removed. 
The  lateral  walls  (not  the  medial)  of  the  nasal 
caxdties  produce  three  curved  folds  one  above 
another;  they  are  concave  below,  and  in  them  the 
conchae  [turbinate  bones]  develop.  The  nasal 
mucosa  covers  these  and  extends  into  the  sphe- 
noid, maxillary,  and  frontal  sinuses,  and  the 
ethmoidal  cells.  The  boundary  between  the 
epithelium  of  the  nasal  pit  and  that  of  the 
pharynx  early  disappears,  and  the  extent  of 
each  in  the  adult  is  uncertain.  Presumably  the 
olfactory  neuro-epithehum  is  derived  from  the 
nasal  pit.  In  man  the  olfactory  region  is  limited 
to  the  superior  and  middle  concha  and  the  part 
of  the    septum   which   is  opposite  them.     This 

regio  olf actor ia  is  covered  by  a  yellowish-brown  membrane  which  may  be 
distinguished  macroscopically  from  the  reddish  mucosa  of  the  regio  res- 
piratoria.  The  latter  includes  the  remainder  of  the  nose.  The  two  re- 
gions may  be  considered  in  turn. 

The  vestibule  or  cavity  of  the  projecting  cartilaginous  portion  of  the 
nose  is  a  part  of  the  respiratory  region  which  is  lined  \vith  a  continuation 
of  the  skin.  Its  stratified  epithehum  has  squamous  outer  cells  and  rests 
upon  a  tunica  propria  with  papillae.  It  contains  the  sheaths  of  coarse 
hairs  (vibrissae)  together  with  numerous  sebaceous  glands.  The  extent 
of  the  squamous  epithehum  is  variable;  frequently  it  is  found  on  the 
middle  concha,  less  often  on  the  inferior  concha. 

The  remainder  of  the  respiratory  mucosa  consists  of  a  pseudo-stratified 
epithelium  with  several  rows  of  nuclei.     It  may  contain  few  or  many 


Fig.  445. — The  Roof  of  the 
Mouth  of  a  Human  Em- 
bryo of  8  Weeks.  X  4- 
(After  Kollmann.) 

na,  Naris ;  ch,  choana;  al.  p., 
i.  p.,  and  pa.  p.,  alveolar, 
intermaxillary,  and  palate 
processes. 


396 


HISTOLOGY. 


goblet  cells.  The  tunica  propria  is  well  developed,  being  even  4  mm. 
thick  on  the  inferior  concha.  It  consists  of  fibrillar  tissue  with  many  elastic 
elements  especially  in  its  deeper  layers.  Beneath  the  epithelium  it  is  thick- 
ened to  form  a  homogeneous  membrana  propria  perforated  with  small  holes. 
L\TTiphocytes  are  present  in  variable  quantity,  sometimes  forming  sohtary 
nodules  and  often  entering  the  epithehum  in  great  numbers.     Branched 


'■■SimsK^'*^.  ^  . 


Ei>ilheliutn. 


Tunica 
propria. 


N'eiii. 


Mucous 
cells. 


Serous 

cells. 


::;*c:<« 


■_\-  ■r^>.^>i«?.; 


Fig.  446.— Vertical  Section  through  the  Mucosa  of  the  Inferior  Conxiia  oe  Man.  X  4S. 
On  the  left  is  a  funnel-shaped  depression  receiving  an  excretory  duct;  nearby  on  the  right  is  the 
section  of  a  large  vein. 

alveolo-tubular  mixed  glands  extend  into  the  tunica  propria.  Their  serous 
portions  have  intercellular  secretory  capillaries.  Both  mucous  and  serous 
cells  contain  a  trophospongium.  The  glands  often  empty  into  funnel 
shaped  depressions  which  are  macroscopic  on  the  inferior  concha,  and 
are  lined  with  the  superficial  epithelium.  The  mucosa  of  the  seA-eral 
paranasal  sinuses  is  thin  ( — 0.02  mm.),  with  less  elastic   tissue  and  but 


NOSE 


397 


few"'  small  glands.  A  pocket  which  extends  into  the  lovrer  part  of  the 
median  septum  and  is  named  the  vomero-nasal  organ  [Jacobson's  organ], 
is  in  man  the  rudimentary  remnant  of  an  important  sense  organ  supplied 


'^^J, 


f 


&b. 


Fig.  447. — Isolated  Cells  of  the  Ol- 
factory Mucosa  of  a  Rabbit.    X  360. 

St,  Supporting  cells  ;  s,  extruded  mucus  re- 
sembling cilia  ;  r,  olfactorj-  cells,  from 
r',  the  lower  process  has  been  torn  ofi ; 
f.  ciliated  cell ;  b,  cells  of  olfactor>- 
glands. 


!  Tunica  propria. 


a-^^i^A'.^^^^^^^i,r\  Epithelium. 

ar- 

h — . 


Fig.  44S. — ^\'erticjVL  Sectiox  of  the  Olfactory 
Mucosa  of  a  Rabbit.    X  50. 

zo.  Zone  of  oval ;  zr,  zone  of  round  nuclei ;  dr,  olfac- 
torj'  glands  ;  a,  excretor>-  duct ;  k,  body  :  g,  fun- 
dus ;  n,  sections  of  the  olfactor>'  nen^es  ;  v,  vein  ; 
ar,  artery  ;  b,  connective  tissue. 


by  the  olfactor}-  nerves.  From  the  fifth  month  of  fetal  hfe  it  is  hned  with 
a  tall  columnar  epithehum  which  is  not  olfactory. 

In  the  regio  oljactoria  the  mucosa  consists  of  a  tunica  propria  and 
an  olfactory  epitheliiun.  The  latter  consists  of  sustentacular  cells  and 
olfactory  cells.  The  super- 
ficial halves  of  the  susten- 
tacular cells  are  cylindrical, 
and  contain  yellowish  pig- 
ment together  T\ath  small 
mucoid  granules  often  ar-* 
ranged  in  vertical  rows 
(Fig.  447 J.  The  more  slen- 
der lower  halves  have  den- 
tate or  notched  borders, 
and  branched  basal  ends 
which  unite  with  those  of 
neighboring  cells  thus  form- 
ing a  protoplasmic  network. 

Their  nuclei,  generally  oval,  are  in  one  plane  and  in  vertical  sections 
they  form  a  narrow  "zone  of  oval  nuclei"  (Figs.  448  and  450).  The 
olfactory  cells  generally  have  round  nuclei  containing  nucleoli.  They 
occur  at  different  levels  and  so  form  a  broad  "zone  of  round  nuclei." 
From  the  protoplasm  which  is  gathered  immediately  about  the  nucleus. 


Epithelium. 


Tunica 
propria. 


Fibers  of  the 
olfactory-  ner\"e. 


Centripetal  process 
of  an  olfactorv  cell. 


Fig.  449. — GoLGi  Prep.^ratiox  of  the  Olf.actory  Region 
OF  .A.  Young  Rat. 


398 


HISTOLOGY. 


each  olfactory  cell  sends  a  slender  cylindrical  process  towards  the  surface, 
where  it  terminates  in  small  hairs.  Basally  the  olfactory  cells  pass  directly 
into  the  axis  cylinders  of  the  olfactory  nerves  (Fig.  449).  Thus  they  are 
ganglion  cells,  their  basal  processes  being  neuraxons.  Cells  intermediate 
between  the  olfactory  and  sustentacular  forms  may  be  found.  At  the  free 
surface  of  the  olfactory  epithehum  there  are  terminal  bars,  and  small 
masses  of  mucus  sometimes  suggesting  cilia.  The  mucus  is  the  product 
of  the  sustentacular  cells.  Near  the  tunica  propria  there  is  a  network  of 
so-called  "basal  cells"  (Fig.  450). 

The  tunica  propria  is  a  network  of  coarse  fibrous  tissue  and  fine 


Wandering  cell. 


Mucus. 

A 


■  ^'"'S^f^Y'— Pigment  granules. 


I 


4' 


Oval  nucleus  of  a 
sustentacular  cell. 


Round  nucleus  of 
an  olfactory  cell. 


Sections  of  olfactory  glands. 
Dilated  duct.  Mucus. 

Fig.  450. — Vertical  Section  through  the  Olfactory  Rkciion  ok  an  Adult.     X  400. 


elastic  fibers  associated  with  many  connective  tissue  cells.  In  some  animals 
(for  example,  the  cat)  it  forms  a  structureless  membrane  next  to  the  epi- 
thelium. It  surrounds  the  numerous  olfactory  glands  [Bowman's  glands]. 
In  man  these  are  branched  cavities  consisting  of  excretory  ducts  extending 
through  the  epithehum,  and  of  gland  bodies  beneath.  ObHque  sections 
of  the  ducts  have  been  mistaken  for  ''olfactory  buds."  The  glands  in  man 
appear  to  be  serous  but  they  sometimes  contain  mucus  in  small  cjuantity. 
They  are  found  not  only  in  the  olfactory  region  but  also  in  the  adjoining 
part  of  the  respiratory  region. 


NOSE.  399 

The  nerves  of  the  nasal  mucosa  consist  of  groups  of  non-medullated 
olfactory  fibers,  which  unite  in  larger  bundles  in  the  tunica  propria  and  pass 
through  the  lamina  cribrosa  of  the  ethmoid  to  enter  the  olfactory  bulb. 
They  are  covered  by  prolongations  of  the  dura.  Medullated  branches  of 
the  trigeminal  nerve  occur  both  in  the  olfactory  and  respiratory  mucosa. 
After  losing  their  myelin  they  form  terminal  ramifications  in  the  tunica 
propria  and  may  ascend  into  the  epithelium.  Thus  they  differ  from  the 
olfactory  fibers  which  generally  do  not  branch. 

The  arteries  are  found  in  the  deeper  layers  of  the  tunica  propria, 
and  they  form  a  thick  capillary  network  just  beneath  the  epithelium.  The 
veins  are  very  numerous,  especially  at  the  inner  end  of  the  inferior  concha 
where  the  tunica  propria  resembles  cavernous  tissue.  Lymphatic  vessels 
form  a  coarse  meshed  network  in  the  deeper  connective  tissue.  Injec- 
tions of  the  subarachnoid  space  follow  the  perineural  sheaths  of  the 
olfactory  nerves  into  the  nasal  mucosa. 


PART  II. 


THE   PREPARATION   AND   EXAMINATION 
OF  MICROSCOPICAL  SPECIMENS. 


The  following  directions  are  limited  to  those  of  fundamental  import- 
ance which  are  hkely  to  be  employed  by  students  who  are  beginning  their 
histological  studies.  Further  information  may  be  obtained  from  "The 
Microtomist's  Vade-mecum"  by  A.  B.  Lee  (3d  ed.,  1903,  Blakiston, 
Philadelphia)  and  from  Mallory  and  Wright's  "Pathological  Technique" 
(3d  ed.,  1904,  Saunders,  Philadelphia).  The  latter  is  particularly 
adapted  to  the  needs  of  medical  students. 

Fresh  Tissues. 
Certain  transparent  tissues  may  be  studied  advantageously  in  a  fresh 
condition.  They  are  merely  spread  in  a  thin  layer  upon  a  clean  glass 
slide,  and  after  a  drop  of  tap  water  and  then  a  clean  cover  glass  have  been 
placed  upon  them,  they  are  ready  for  the  miscrocope.  (The  glass  shdes 
and  covers  are  to  be  washed  with  water,  using  soap  if  necessary,  and 
sometimes  alcohol  or  strong  acids,  but  all  trace  of  these  must  be  removed. 
Linen  cloths,  because  of  their  small  quantity  of  Hnt,  are  the  proper  towels 
for  drying  the  glassware.  Covers  and  shdes  as  received  from  the  dealers 
are  never  ready  for  use,  and  some  M^hich  remain  hazy  after  thorough  wash- 
ing are  worthless.)  The  fresh  tissue  is  spread  upon  the  slide  with  needles, 
being  'teased'  into  small  fragments  or  drawn  out  into  a  thin  film.  Pure 
water  causes  some  swelhng  of  the  tissue  so  that  dilute  solutions  of  common 
salt  are  preferable.  A  0.6  per  cent,  solution  has  recently  been  found  to 
cause  less  distortion  than  the  somewhat  stronger  solutions  formerly  recom- 
mended. The  tissue  having  been  spread  in  the  center  of  the  slide  and  a 
drop  or  two  of  salt  solution  placed  upon  it,  the  cover  glass  is  lowered  so  that 
air  bubbles  are  not  caught  beneath  it.  Especially  with  the  larger  shdes 
which  are  to  be  preserved  permanently  this  should  be  done  as  follows.  The 
square  or  oblong  cover  glass  is  held  over  the  specimen  and  its  left  edge  is 
first  brought  in  contact  with  the  shde;  a  needle  held  in  the  left  hand 
keeps  this  edge  in  position.     Another  needle  held  in  the  right  hand  with  its 

400 


FRESH    TISSUES.  4OI 

point  beneath  the  right  edge  of  the  cover  enables  one  to  have  perfect  con- 
trol of  it  while  it  is  being  lowered.  The  contact  between  the  cover  and  the 
mounting  medium  (salt  solution  in  this  case)  spreads  gradually  from  left 
to  right  as  the  cover  is  lowered,  expelling  the  air  as  it  advances.  If  bubbles 
are  caught  in  the  medium,  the  cover  may  be  alternately  raised  and  lowered 
a  Kttle  until  they  escape,  but  once  the  cover  is  fiat  upon  the  specimen  it 
should  not  be  lifted. 

Connective  tissue,  meduUated  nerves,  fat,  desquamated  epithelial 
cells  and  blood  should  be  examined  in  the  fresh  state  by  every  student  as 
showing  certain  features  better  than  the  preserved  specimens.  Chorionic 
villi  may  be  identified  in  this  way,  and  the  cells  in  urine  are  studied  un- 
stained. A  drop  of  acetic  acid  (from  i  to  5  per  cent.)  placed  upon  connective 
tissue  causes  the  white  fiber  to  swell  and  disintegrate,  but  renders  the  elas- 
tic tissue  and  the  nuclei  more  distinct.  A  few  drops  of  stain  may  be  placed 
upon  the  tissue  for  some  minutes  and  then  washed  off  in  order  to  bring 
out  the  nuclei.  Methylene  blue  (i  per  cent,  aqueous  solution)  and  methyl 
green  (i  per  cent,  solution  in  20  per  cent,  alcohol)  or  the  haematoxyline 
solutions  may  be  used  for  this  purpose.  If  sections  are  overstained  a  more 
dilute  solution  or  shorter  application  is  indicated,  but  if  the  section  is  pale, 
prolonged  staining  or  stronger  solutions  are  required.  Thus  the  time 
limits  given  with  the  various  dyes  are  only  approximate  as  the  response  of 
different  tissues  is  not  uniform,  and  different  samples  of  a  given  solution 
vary  in  their  staining  capacity. 

Isolation. 

Some  tissues  cannot  properly  be  separated  into  their  elements  in  the 
fresh  condition  but  may  be  shaken  or  teased  apart  after  preliminary 
treatment.  Epithelial  cells  become  separable  after  remaining  from  5  to 
24  hours  in  33  per  cent,  alcohol  (40  cc.  of  95  per  cent,  alcohol  and  60  cc. 
of  water).  The  pieces  of  epithelium  used  should  be  small  (5-10  mm. 
square).  The  same  treatment  prolonged  for  one  or  two  weeks  is  employed 
in  isolating  the  nerve  cells  of  the  spinal  cord.  Muscle  cells  may  be  pulled 
apart  after  remaining  some  hours  in  a  fresh  35  per  cent,  solution  of  potas- 
sium hydrate.  The  muscle  fibers  should  be  examined  in  a  few  drops  of  the 
same  solution,  since  they  disintegrate  if  it  is  diluted.  They  may  however  be 
transferred  to  solutions  of  potassium  acetate  which  neutrahzes  the  potash 
and  prevents  further  maceration.  The  elements  of  nails  may  be  scraped 
off  from  fragments  boiled  in  a  test  tube  containing  a  concentrated  solution 
of  potassium  hydrate.  Immersion  in  cold  concentrated  sulphuric  acid  is 
recommended  for  the  same  purpose. 

Another  solvent  for  the  intercellular  subtances  of  muscle  is  a  saturated 
26 


402  HISTOLOGY. 

solution  of  potassium  chlorate  in  nitric  acid.  (About  5  gr.  of  potassium 
chlorate  should  be  added  to  20  cc.  of  nitric  acid.)  The  muscle  fibers  should 
be  separable  in  from  i  to  6  hours.  They  should  be  washed  in  distilled 
water  for  an  hour  or  a  few  days  so  as  to  remove  the  acid,  and  then  may  be 
examined  in  water  or  in  glycerine. 

Other  macerating  fluids  are  10  to  20  per  cent,  nitric  acid,  diluted  either 
with  water  or  with  salt  solution ;  ^-g-g-  to  i  of  i  per  cent,  of  chromic  acid ; 
and  water,  by  which  the  pulpy  portion  of  organs  may  be  removed  from  the 
connective  tissue  framework.  Complex  but  valuable  methods  for  demon- 
strating the  connective  and  reticular  networks  have  been  described  by 
Mall  and  FHnt.  They  involve  digestion  of  the  tissues  with  pancreatic 
extract. 

Sectioning  Fresh  Material. 

Since  the  cutting  of  freehand  sections  of  fresh  tissue  held  between 
pieces  of  pith  is  no  longer  practised,  the  most  rapid  method  for  obtaining 
sections  is  by  means  of  the  freezing  microtome.  Small  blocks  of  fresh  tissue 
not  over  5  mm.  thick  are  moistened  with  water  and  placed  upon  the  carrier 
of  the  microtome,  where  they  are  frozen  by  a  jet  of  carbon  dioxide  pro- 
ceeding from  a  cyhnder  of  the  liquefied  gas.  Sections  10-15  /"  thick  may 
be  chiselled  from  the  frozen  tissue  and  placed  in  a  dish  of  water,  in  which 
they  unroll.  Then  they  are  floated  upon  a  sUde  and  may  be  stained  by 
ordinary  methods.  Frozen  sections  may  be  made  from  tissue  hardened  in 
formaline  as  well  as  from  fresh  material.  In  some  cases  this  method  is  of 
special  value  in  studying  normal  tissue ;  for  rapid  diagnosis  of  pathological 
conditions  it  is  indispensable. 

Descriptions  of  the  freezing  and  other  microtomes  with  full  directions 
for  their  use  will  be  found  in  Mallory  and  Wright's  **  Pathological  Tech- 
nique." The  use  of  the  instruments,  however,  is  seldom  learned  except  by 
personal  demonstration  in  the  laboratory. 

Fixation. 
The  fixation  of  tissues  is  the  process  by  which  post  mortem  changes  are 
prevented,  mitosis,  for  example,  being  checked  at  once  and  the  mitotic 
figure  permanently  preserved.  The  hardening  of  the  tissue  is  completed 
subsequently  by  immersion  in  alcohol.  Small  blocks  of  the  desired  tissue 
(about  I  cc.  in  volume  and  preferably  less  than  i  cm.  thick)  should  be 
dropped  without  handhng  into  a  considerable  quantity  of  the  fixing  fluid. 
Contact  between  the  fingers  and  the  peritonaeum  is  sufficient  to  destroy 
the  thin  mesothehum.  It  is  often  advisable  to  place  a  piece  of  absorbent 
cotton  beneath  the  tissue  so  that  the  fixing  fluid  may  have  access  to  its 


nXIXG     FLUIDS.  403 

lower  surface.  Tubular  organs  should  be  cut  open  before  being  put  in  the 
fluid,  and  their  contents  together  with  blood  upon  the  surface  of  the  block 
may  be  washed  away  with  salt  solution.  Membranes  may  be  kept  flat  and 
smooth  by  being  tied  across  the  end  of  a  short  tube  or  a  detached  bottle 
neck.  After  being  used  once  the  fixing  fluids  should  be  thrown  away,  ex- 
cept alcohol,  which  can  be  put  to  other  uses.  The  follo^dng  mixtures  are 
those  most  frequently  used. 

Zenker^ s  Fluid  is  kept  m  stock  as  glacial  acetic  acid  and  the  following 
solution,  in  preparing  which  the  water  is  heated  and  the  ingredients  are 
stirred  with  a  glass  rod.  (Metal  instruments  should  not  be  put  in  Zenker's 
fluid.; 

Bichromate  of  potassium 25  gr. 

Sodium  sulphate 10  gr. 

Mercuric  chloride  (corrosive  sublimate) 5°  gr- 

Water 1000  cc. 

Shortly  before  using,  Zenker's  fluid  is  to  be  completed  by  adding  5  cc. 
of  glacial  acetic  acid  to  100  cc.  of  the  solution.  The  blocks  of  tissue  placed 
in  it  should  be  from  4  to  6  mm.  thick;  after  remaining  in  the  fluid  from  10 
to  24  hours  they  are  to  be  placed  in  running  water  for  in  water  frequently 
changed)  for  the  same  length  of  time.  Then  they  are  transferred  to  80  per 
cent,  alcohol. 

The  transfer  of  tissues  from  water  to  alcohol  or  vice  versa  is  one  of  the 
commonest  procedures.  The  abrupt  change  from  water  to  strong  alcohol 
causes  \dolent  diffusion  currents  which  may  distort  the  tissues;  therefore 
graded  percentages  of  alcohol  are  used,  50  per  cent.,  70  per  cent.,  80  per 
cent.,  95  per  cent.,  and  absolute  alcohol  being  always  at  hand.  (Sometimes 
90  per  cent,  also  is  used.;  These  may  be  prepared  from  the  commercial 
95  per  cent,  alcohol  by  adding  water  in  the  foUowing  proportions: 

Ninety   per  cent., — 475  cc.  of  95  per  cent,  and  25  cc.  of  distilled  water. 
Eighty    per  cent., — 425  "         "         "  "     75  "  "  " 

Seventy  per  cent., — 370  "         "         "  "  130  "  "  " 

Fifty       per  cent., — 265  "         "         "  "  235   "  "  " 

Tissues  may  generally  be  transferred  between  water  and  50  per  cent,  al- 
cohol without  injury.  In  passing  from  50  per  cent,  to  absolute  they  may  be 
placed  successively  in  70  per  cent.,  80  per  cent.,  and  95  per  cent.,  remaining 
in  each  only  long  enough  to  become  saturated.  Stains  may  be  rated  accord- 
ing to  the  alcohol  they  contain;  the  transition  from  80  per  cent,  to  an  aque- 
ous stain  should  be  graded  as  from  80  per  cent,  to  water.  It  is  a  general 
principle  that  all  these  transfers  should  be  gradual  for  the  best  results. 
Nevertheless  abrupt  transitions  are  often  made,  and  ordinarily  the  tissue 
preserved  in  Zenker's  fluid  and  washed  in  water  is  next  immersed  in  80 
per  cent  alcohol. 


404  HISTOLOGY. 

The  chief  fault  of  Zenker's  fluid  is  its  tendency  to  form  a  precipitate  of 
mercuric  chloride  (corrosive  sublimate)  within  the  tissue.  The  precipitate 
is  dissolved  out  by  the  addition  of  enough  tincture  of  iodine  to  the  80  per 
cent,  alcohol  to  give  it  a  mahogany  color.  The  color  fades  as  the  iodine 
combines  with  the  sublimate  and  it  should  be  renewed  until  for  two  days 
there  is  no  perceptible  change  in  its  color.  This  may  require  a  week  or  more. 
Then  the  tissue  is  transferred  to  80  per  cent,  alcohol  which  is  renewed  as 
long  as  it  becomes  discolored  by  the  iodine.  In  80  per  cent,  alcohol  the 
tissue  may  remain  for  months  but  it  gradually  deteriorates.  The  pro- 
longed action  of  iodine  causes  some  loss  in  staining  capacity;  nevertheless 
the  treatment  with  iodine  is  an  essential  routine  part  of  this  method  of 
fixation,  and  it  should  be  thorough  enough  to  remove  the  precipitate. 
The  latter  appears  in  sections  as  dark  blotches  resembling  pigment.  They 
may  be  dissolved  after  sections  have  been  cut  and  attached  to  the  sUde  by 
immersing  the  sHde  in  the  iodine  solution  and  then  rinsing  it  in  80  per 
cent,  alcohol. 

Telly esnizcky's  Fluid  is  employed  like  Zenker's  fluid  but  since  it  con- 
tains no  mercuric  chloride,  the  after-treatment  with  iodine  is  unnecessary. 
This  fluid  is  a  3  per  cent,  aqueous  solution  of  bichromate  of  potassium  to 
which  glacial  acetic  acid  should  be  added  shortly  before  using  (5  cc.  of 
acetic  acid  to  100  cc.  of  the  solution).  Tissues  may  remain  in  it  for  two 
or  more  days.  The  reagent  is  washed  out  in  running  water,  and  the  tissue 
is  transferred  to  80  per  cent,  alcohol. 

Formaline  is  a  40  per  cent,  aqueous  solution  of  formaldehyde  gas. 
Ten  per  cent,  aqueous  solutions  of  formahne,  which  are  4  per  cent,  solu- 
tions of  formaldehyde,  are  used  for  the  preservation  of  small  embryos  and 
of  various  tissues.  Small  human  embryos  obtained  by  practitioners  should 
he  placed  at  once  i?i  10  per  cent,  formaline  and  forwarded  to  an  embryological 
laboratory.  Tissues  should  remain  in  the  10  per  cent,  formahne  for  24 
hours  or  somewhat  longer,  and  then  are  transferred  to  80  per  cent,  alcohol 
in  which  they  generally  shrink.  (Frozen  sections  may  be  made  from  the 
material  taken  directly  from  formaline  and  rinsed  in  water.)  Instead  of 
transferring  the  tissue  from  the  formaline  to  80  per  cent,  alcohol,  some 
histologists  recommend  placing  it  at  once  in  absolute  alcohol  for  2  days, 
after  which  it  is  immersed  in  80  per  cent.  Formaline  is  used  as  a  fixing 
agent  in  many  solutions,  especially  the  following. 

Orth's  Fluid  is  Muller's  Fluid  with  the  addition  of  formahne.  Muller's 
fluid  is  a  slow  fixing  solution,  in  large  quantities  of  which  objects  may  be 
left  from  i  to  6  weeks ;  after  wasliing  4  to  8  hours  in  running  water  they  are 
put  through  graded  alcohols  in  which  the  tissue  is  hardened ;  or  the  tissue 
may  be  both  fixed  and  hardened  by  remaining  in  the  fluid  for  six  months. 


DECALCIFYING    FLUIDS.  405 

It  is  a  solution  of  30  grams  of  sodium  sulphate  and  60  grams  of  potassium 
bichromate  in  3000  cc.  of  water.  To  make  Orth's  fluid,  10  cc.  of  formahne 
are  added  to  100  cc.  of  Mliller's  fluid  shortly  before  using.  Small  blocks 
of  tissue  should  remain  in  it  for  3  or  4  days,  when,  after  washing  thoroughly 
in  running  water,  they  are  put  in  80  per  cent,  alcohol. 

Alcohol.  The  higher  grades  of  alcohol  are  important  fixing  fluids, 
although  for  most  purposes  inferior  to  Zenker's  fluid  or  formahne.  Tissue 
may  be  put  directly  into  95  per  cent,  or  absolute  alcohol,  a  piece  of  ab- 
sorbent cotton  being  under  it.  The  alcohol  should  be  changed  after  3  or 
4  hours,  and  after  3  or  4  days  the  tissue  is  transferred  to  80  per  cent. 
Some  histologists  recommend  passing  the  fresh  tissue  through  graded  alco- 
hols before  putting  it  in  absolute;  this  causes  less  shrinkage  but  is  said  to 
fix  imperfectly.     One  may  begin  with  80  per  cent. 

Specimens  should  be  kept  in  80  per  cent,  or  90  per  cent,  alcohol  after 
they  have  been  preserved.  They  macerate  in  the  weaker  alcohols  and  lose 
their  staining  capacity  in  those  which  are  stronger. 

Decalcification. 

Specimens  which  contain  bone  or  calcareous  material  cannot  be 
sectioned  until  they  have  been  decalcified,  which  can  be  done  only  after 
they  have  been  fixed,  and  hardened  for  a  few  days  in  alcohol.  They  are 
then  placed  in  considerable  amounts  of  dilute  nitric  acid  (3  to  5  cc.  of  con- 
centrated nitric  acid  in  100  cc.  of  water).  This  should  be  renewed  for  3  or 
4  days,  imtil  the  bone  can  be  cut  with  a  scalpel  or  be  penetrated  easily  with 
a  needle.  The  acid  is  removed  from  the  tissue  by  inmiersion  in  running 
water  for  a  day,  and  the  block  is  returned  to  the  alcohol. 

Phloroglucin  is  sometimes  added  to  the  decalcifying  fluid  to  protect  the 
tissue,  and  the  nitric  acid  may  be  diluted  with  alcohol.  The  following 
solution  has  been  recommended: 

Phloroglucin i 

Nitric  acid 5 

Alcohol 70 

Water 30 

A  sHght  addition  (i  or  2  per  cent.)  of  nitric  or  hydrochloric  acid  to 
80  per  cent,  alcohol  may  be  used  in  decalcifying  email  embryos. 

Imbeddeng. 

Imbedding  is  the  process  by  which  blocks  of  fiixed,  hardened,  and 

decalcified  tissues  are  prepared  for  sectioning.     Sometimes  the  tissue  is 

stained  before  being  imbedded,  as  will  be  described  later;    often  all  the 

staining  is  done  after  the  sections  have  been  cut.     Imbedding  consists  in 


4o6  HISTOLOGY. 

surrounding  and  infiltrating  the  tissue  with  a  firm  substance  which  can 
readily  be  cut  into  thin  sections.  Celloidin  and  paraffin  are  used,  each 
having  its  pecuhar  advantages. 

To  imbed  in  celloidin  one  needs  graded  alcohols,  a  mixture  of  equal 
parts  of  ether  and  absolute  alcohol,  a  thin  and  a  thick  solution  of  celloidin, 
and  vulcanized  fiber  blocks  of  such  size  as  can  be  clamped  in  the  carrier  of 
the  microtome. 

Thick  celloidin  consists  of  30  grams  of  Schering's  dry  granular  celloidin 
dissolved  in  from  200  to  250  cc.  of  an  equal  mixture  of  ether  and  absolute 
alcohol.  It  has  a  thick  syrupy  consistency  and  becomes  constantly  denser 
as  the  ether  evaporates.  It  should  be  kept  in  a  tightly  closed  preserve  jar. 
Thin  celloiden  contains  twice  as  much  "ether  and  absolute"  as  the  thick. 

The  piece  of  hardened  tissue  is  trimmed  to  the  size  and  shape  desired 
and  is  put  successively  in  95  per  cent.,  absolute,  and  absolute  and  ether, 
remaining  24  hours  in  each.  Then  it  is  immersed  in  thin  celloidin  and 
finally  in  thick  celloidin,  in  each  of  which  it  stays  from  24  hours  to  a  week 
or  even  longer.  The  success  of  the  process  depends  largely  upon  the 
thorough  penetration  of  the  celloidin  into  the  tissue.  The  time  required 
in  the  celloidin  varies  with  the  penetrability  of  the  tissue  and  the  size  of  the 
piece.  After  remaining  in  the  thick  celloidin  long  enough  the  tissue  is 
taken  out  with  a  mass  of  adherent  celloidin  and  is  pressed  gently  against 
the  roughened  surface  of  a  block  of  vulcanized  fiber.  The  celloidin  should 
cover  the  tissue  and  spread  out  at  its  base  upon  the  block.  As  soon  as  a 
film  has  formed  over  its  surface,  the  block  and  attached  specimen  are  dropped 
into  80  per  cent,  alcohol  in  which  the  mass  becomes  firm.  It  is  ready  for 
sectioning  in  6  hours.  While  the  block  is  clamped  in  the  sliding  microtome 
with  which  sections  from  10  to  15  ii  should  be  cut,  it  is  kept  moistened  with 
80  per  cent,  alcohol ;  the  knife  also  should  be  wet  with  the  same.  Sections 
are  immediately  transferred  to  a  dish  of  80  per  cent,  alcohol  in  which  they 
unroll,  and  where  they  remain  until  it  is  desired  to  stain  them.  Each  sec- 
tion is  surrounded  by  celloidin  which  it  is  not  desirable  to  remove;  the 
sections  would  then  be  too  fragile.  Therefore  they  are  not  to  be  placed  in 
absolute  alcohol.  In  case  the  tissue  was  not  properly  imbedded  it  may  be 
returned  to  ether  and  absolute,  and  again  be  put  through  the  celloidins. 

To  imbed  in  paraffin  the  block  of  hardened  tissue  is  immersed  for 
from  6  to  12  hours  in  the  following  fluids  successively:  95  per  cent.;  ab- 
solute; a  mixture  of  equal  parts  of  chloroform  and  absolute;  chloroform. 
Then  it  is  transferred  to  chloroform  saturated  with  paraflin,  which  may  be 
kept  warm  by  placing  on  top  of  the  paraffin  bath;  in  this  mixture  it  re- 
mains about  4  hours  and  then  is  put  in  melted  paraffin.  Hard  paraffin 
which  melts  at  50°  is  ordinarily  used,  but  if  this  is  brittle  when  cut  into  the 


IMBEDDING.  407 

microscopic  sections  at  the  temperature  of  the  room,  a  grade  with  a  lower 
melting  point  should  be  used.  The  melted  paraffin  should  be  in  a  paraffin 
bath  or  in  a  thermostat  maintained  at  a  temperature  but  sHghtly  above  the 
melting  point  of  the  paraffin.  The  tissue  should  not  remain  in  hot  paraffin 
longer  than  is  required;  it  is  generally  left  2  hours  in  one  cup  and  then 
is  transferred  to  another  in  which  it  remains  for  two  hours  longer.  The 
purpose  of  this  transfer  is  to  free  the  tissue  from  chloroform,  most  of 
which  remains  in  the  first  cup. 

The  imbedding  frame  in  which  the  paraffin  is  to  be  cooled,  is  a  box  the 
bottom  of  which  is  made  by  a  glass  plate  and  the  sides  of  which  are  of  metal 
in  two  L  shaped  pieces.  By  sliding  the  latter  back  and  forth  in  relation  to 
one  another,  the  size  of  the  space  which  they  enclose  can  be  varied.  Before 
using  the  frame  the  inside  surfaces  of  the  metal  pieces  together  with  that 
surface  of  the  glass  on  which  they  rest  are  rubbed  with  glycerine  and  the 
frame  is  warmed  by  placing  it  for  a  few  minutes  on  the  top  of  the  paraffin 
bath.  Melted  paraffin  is  then  poured  into  it,  and  the  tissue,  removed  from 
the  cup  by  means  of  a  spatula,  is  added.  It  sinks  to  the  bottom  and  may 
be  placed  in  any  desired  position  by  means  of  needles  warm  enough  to 
prevent  the  paraffin  from  solidifying  over  their  surface.  The  paraffin  is 
then  quickly  cooled  by  lowering  the  frame  into  a  basin  of  cold  water  so 
that  the  latter  surrounds  the  metal  pieces.  Water  must  not  reach  the  upper 
surface  of  the  paraffin  until  it  has  solidified;  then  the  frame  is  placed  under 
water  and  in  a  few  minutes  the  glass  plate  and  metal  pieces  may  be  detached 
from  the  solid  paraffin.  As  soon  as  it  is  thoroughly  cool  it  may  be  sec- 
tioned. 

Before  the  imbedded  object  is  attached  to  a  block  of  vulcanized  fiber, 
superfluous  paraffin  is  cut  away  leaving  the  tissue  rising  from  a  broad 
base  of  paraffin  and  completely  surrounded  by  a  thin  layer.  The  base 
is  placed  upon  a  heated  spatula  which  rests  upon  the  fiber  block.  When 
the  paraffin  has  melted  somewhat,  the  spatula  is  withdrawn  and  the  base 
is  pressed  down  upon  the  block,  to  which  it  adheres  securely  when  the  par- 
affin has  solidified  again.  The  fiber  block  is  then  clamped  in  a  "precision" 
microtome.  If  a  rotary  microtome  is  used  the  paraffin  is  attached  to  a  metal 
disc  in  place  of  a  fiber  block.  Sections  should  be  from  6  to  lo  /^  thick,  but 
under  favorable  conditions  they  may  be  made  2  ij.  thick.  If  the  paraffin  on 
both  sides  of  the  tissue  is  trimmed  parallel  with  the  knife  blade,  the  succes- 
sive sections  adhere  to  one  another  by  their  edges  forming  ribbons.  Thus 
the  sections  may  easily  be  kept  in  order.  The  first  one  cut  is  attached 
to  the  upper  left  hand  comer  of  the  slide,  and  the  others  follow  Hke  Hnes 
upon  a  printed  page.  Sections  mounted  in  this  way  are  called  serial 
sections.    Paraffin  sections,  as  they  are  taken  from  the  microtome,  are  laid 


4o8  HISTOLOGY. 

in  shallow  boxes.  Before  being  stained  they  must  be  attached  to  shdes  as 
follows. 

To  attach  paraffin  sections  to  a  slide,  a  mixture  of  equal  parts  of  glyc- 
erine and  white  of  egg  is  used,  which  may  conveniently  be  called  albumen. 
Its  two  ingredients  should  be  stirred  together  thoroughly  and  filtered,  after 
which  a  small  lump  of  camphor  is  added  as  a  preservative.  It  is  kept  in  a 
capped  bottle  with  a  glass  rod  for  a  dropper.  A  drop  or  two  are  placed 
upon  a  clean  shde  and  rubbed  evenly  with  the  finger  over  all  that  area  upon 
which  sections  may  be  placed.  It  should  be  free  from  bubbles  and  should 
make  a  layer  thick  enough  to  allow  the  finger  to  glide  easily  over  the  surface 
of  the  shde.  Then  a  few  drops  of  water  are  placed  upon  it,  forming  a  layer 
over  the  albumen  deep  enough  to  float  the  paraffin  sections,  strips  of  which 
are  placed  upon  the  water.  The  shiny  side  of  the  ribbon  should  rest  upon 
the  water.  The  slide  is  then  held  for  a  moment  over  the  flame  of  an  alcohol 
lamp  so  that  the  water  is  heated  and  the  sections  become  flat  and  smooth. 
The  paraffin  must  not  be  melted.  This  manipulation  with  a  large  shde 
bearing  several  rows  of  serial  sections,  requires  some  skill;  the  water  should 
not  come  in  contact  with  the  fingers  holding  the  shde  and  if  the  albumen 
layer  ends  abruptly  before  reaching  the  border  of  the  shde,  the  water  will 
not  spread  beyond  it.  Surface  tension  is  such  that  enough  water  can  be 
put  upon  the  shde  to  float  the  sections  freely.  After  the  flattening  process 
the  water  is  cautiously  drained  off  by  a  moist  sponge  held  at  the  corner  of  the 
shde.  The  sections  settle  down  upon  the  albumen  and  may  be  arranged  in 
straight  fines  with  needles  applied  to  the  paraffin  but  not  to  the  sections 
themselves.  After  this  the  slide  is  held  vertically  in  contact  with  filter 
paper  to  drain  off  any  water  which  may  remain.  The  slide  is  then  placed  in 
a  drying  oven  which  is  not  warm  enough  to  melt  the  paraffin.  It  is  well  to 
let  the  sections  remain  there  over  night  but  a  few  hours  may  be  sufficient  to 
dry  them  thoroughly. 

In  preparing  large  numbers  of  slides,  each  bearing  only  one  or  two 
paraffin  sections,  fragments  of  the  ribbon  containing  the  desired  number 
of  sections  are  floated  in  a  basin  of  water  warm  enough  to  flatten  but  not  to 
melt  them.  Slides  rubbed  with  albumen  are  dipped  into  the  water  beneath 
the  sections  which  are  held  in  place  upon  them  with  a  needle.  The  slides 
are  drained  and  dried  in  the  usual  way. 

Staineng  and  Mounting. 
The  staining  oj  paraffin  sections  is  accomplished  by  placing  the  slides 
to  which  the  sections  have  been  attached,  in  pairs  back  to  back,  in  tube- 
like vials  containing  stains.     One  should  have  a  dozen  such  vials  containing 
various  alcohols,  xylol,  stains,  etc.,  the  sections  being  passed  from  one  to  the 


STAINING    AND     MOUNTING.  4^9 

other.  The  reagents  are  kept  tightly  corked  and  can  be  used  for  some  time 
before  being  renewed.  The  separate  stains  are  to  be  described  in  the 
following  section.  For  staining  large  numbers  of  parafiin  sections  pans 
have  been  made  with  vertically  grooved  sides,  resembhng  wooden  shde 
boxes.  In  these  25  or  50  slides  may  be  stained  at  once,  one  fluid  being 
poured  out  of  the  pan  and  another  substituted.  Staining  solutions  can  be 
used  repeatedly  and  are  not  to  be  throT\Ti  away. 

Before  parafiSn  sections  are  stained,  the  paraffin  is  to  be  removed  by 
immersing  the  shde  in  xylol;  it  is  then  transferred  in  turn  to  a  mixture  of 
equal  parts  of  xylol  and  absolute  alcohol,  then  to  absolute,  95  per  cent., 
and  through  graded  alcohols  to  that  which  corresponds  with  the  solvent  of 
the  stain.  After  being  stained  the  sections  must  be  dehydrated,  cleared  and 
mounted.  They  are  dehydrated  in  95  per  cent.,  and  then  in  absolute  alcohol. 
They  are  transferred  to  the  mixture  of  xylol  and  absolute,  and  then  into 
xylol  in  which  they  should  become  perfectly  clear.  Since  the  sections  are 
thin  and  easily  penetrated,  they  need  to  remain  only  a  few  minutes  in  each 
of  these  reagents.  After  the  section  has  been  cleared  the  xylol  is  drained 
from  the  shde,  the  borders  of  which  (up  to  the  specimen)  may  be  wiped 
dry;  the  section  itself  should  not  become  dry  before  a  drop  or  two  of  damar 
is  placed  upon  it  and  the  cover  glass  is  lowered  as  described  under  fresh 
tissue.  The  shde  may  be  used  at  once  although  the  damar  does  not  become 
soHd  for  some  time. 

Damar  is  a  resin  derived  from  trees  of  the  genus  Damara;  for  mount- 
ing microscopic  objects  is  should  be  dissolved  in  xylol  and  filtered.  The 
solution  should  be  perfectly  clear  and  nearly  colorless.  By  evaporation  of 
the  xylol  it  thickens,  but  it  may  be  diluted  at  any  time  by  adding  more  xylol. 
When  ready  for  use  it  should  have  the  consistency  of  rather  thin  syrup. 
Damar  is  preferable  to  balsam  since  the  latter  gradually  becomes  yellow 
after  it  has  been  used. 

The  staining  0}  celloidin  sections  is  performed  in  a  series  of  small 
shallow  staining  dishes.  The  sections  are  taken  from  80  per  cent,  alcohol 
and  transferred  through  graded  alcohols  to  water  or  the  solvent  of  the  stain. 
Then  they  are  immersed  in  the  stains,  washed  in  alcohol  or  water,  dehy- 
drated, cleared,  and  mounted.  They  are  transferred  from  dish  to  dish 
with  bent  metal  or  glass  needles.  Because  celloidin  is  dissolved  in  the 
strongest  alcohols,  the  sections  are  dehydrated  in  95  per  cent.  Since  this 
extracts  stains  the  sections  are  passed  through  it  rapidly  and  are  placed  in 
the  clearing  fluid,  either  oil  of  bergamot  or  oil  of  origanum  (oleum  origani 
cretici).  In  this  they  should  quickly  become  clear;  if  opaque  spots  remain, 
the  section  may  be  returned  to  95  per  cent,  for  further  dehydration.  The 
clearing  oils  may  be  used  repeatedly  and  are  not  to  be  thro'RTi  away;   the 


41 0  HISTOLOGY. 

alcohol  cannot  be  used  twice.  The  section  is  mounted  by  taking  it  from 
the  oil  upon  a  spatula,  and  transferring  it  to  the  center  of  a  shde  upon  which 
it  should  be  spread  out  flat.  The  oil  around  it  is  wiped  away  and  several 
layers  of  filter  paper  are  placed  directly  upon  the  section;  the  finger  is 
rubbed  over  them  so  that  the  section  is  further  flattened.  Remove  the 
filter  paper,  and  mount  in  damar  as  with  parafifin  sections. 

The  handling  of  large  numbers  of  celloidin  sections  is  facihtated  if 
they  are  placed  in  a  perforated  cup  which  fits  into  another  ordinary  cup. 
The  ordinary  cups  contain  the  various  reagents  and  the  sections  are  trans- 
ferred from  one  to  the  other  in  the  perforated  cup.  The  latter  may  be 
obtained  as  Hobb's  tea  infusers,  and  the  sohd  lemonade  cups  are  of  proper 
size  to  receive  them. 

General  Stains. 

Haematoxyline  and  eosine.  Haematoxyline  is  a  dye  obtained  from 
logwood,  which  stains  nuclear  structures  blue.  Eosine  is  an  aniline  dye 
staining  protoplasm  red.  This  and  all  anihne  dyes  used  in  histological 
stains  should  be  prepared  by  Griibler  in  Germany. 

There  are  many  solutions  of  heamatoxyline  among  which  is  the  fol- 
lowing : 

Haematoxyline  cn-stals i  gr. 

Saturated  aqueous  solution  of  ammonia  alum loo  cc. 

Water 300  cc. 

Dissolve  the  crystals  in  the  water,  which  may  be  heated,  and  add  the  alum 
solution.  Put  the  mixture  in  a  bottle  and  drop  in  a  crystal  of  thymol  to 
prevent  the  growth  of  mould.  A  loose  plug  of  cotton  is  used  for  a  stopper 
and  in  this  condition  the  solution  is  kept  in  the  light  for  10  days  to  ripen. 
It  changes  color  during  this  process  of  oxidation,  after  which  it  is  ready  for 
use  and  is  kept  tightly  stoppered.  It  deteriorates  in  a  few  months.  If  a 
strong  solution  is  desired  the  amount  of  water  may  be  reduced. 

Another  haematoxyline  solution  in  common  use  is  Delafield's.  It  is 
made  by  dissolving  4  gr.  of  haematoxyhne  crystals  in  25  cc.  of  95  per  cent, 
alcohol,  and  then  adding  400  cc.  of  a  saturated  aqueous  solution  of  ammonia 
alum.  This  is  kept  unstoppered  for  3  or  4  days  and  then  is  filtered. 
100  cc.  each  of  95  per  cent,  alcohol  and  of  glycerine  are  added.  It  should 
not  be  used  until  it  has  become  dark  colored  by  remaining  in  the  hght  for 
several  days.     Then  it  is  to  be  filtered  and  tightly  stoppered. 

Eosine  is  sold  in  two  forms,  one  soluble  in  water,  the  other  in  alcohol. 
In  connection  with  the  haematoxyhne  stain,  a  yV  to  i  per  cent,  aqueous 
solution  may  be  used;  or  a  i  per  cent,  solution  of  alcoholic  eosine,  made 
in  60  per  cent,  alcohol. 

To  stain  with  haematoxyline  and  eosine  the  sections  are  placed  in  the 


GENERAL    STAINS.  4II 

haematoxyline  solution  from  2  minutes  to  an  hour.  They  are  then  placed 
in  water  changed  repeatedly  for  half  an  hour  or  longer  (they  may  remain 
in  it  over  night).  As  seen  under  the  microscope  the  nuclei  should  be  deeply 
stained  but  the  protoplasm  should  be  nearly  free  from  color.  Stain  in 
eosine  for  i  to  5  minutes;  dehydrate,  clear,  and  mount.  For  paraffin 
sections  this  means  treatment  with  95  per  cent.,  absolute  and  xylol,  xylol, 
and  damar.  For  celloidin  sections,  95  per  cent.,  oil  of  origanum,  and 
damar. 

Methylene  blue  and  eosine  is  highly  recommended,  especially  for  tissues 
fixed  in  Zenker's  fluid  and  sectioned  in  paraffin.  Stain  in  a  5  or  10  per  cent. 
aqueous  solution  of  eosine  for  20  minutes  or  longer,  overstaining  the  tissue 
since  the  eosine  is  partly  lost  in  the  subsequent  treatment.  Wash  out  the 
excess  of  stain  in  water,  and  transfer  to  Unna's  alkahrie  methylene  blue 
diluted  with  three  or  four  times  as  much  water.  Unna's  blue  is  made  by 
dissolving  i  gr.  of  methylene  blue  and  i  gr.  of  potassium  carbonate  in  100 
cc.  of  water.  Sections  should  be  stained  in  the  diluted  solution  for  10  to 
15  minutes.  Then  they  are  washed  in  water  and  dehydrated  and  decolor- 
ized in  95  per  cent,  alcohol,  moving  the  section  about  so  that  the  stain  may 
be  washed  out  evenly.  The  pink  color  returns  and  when,  as  seen  under 
the  microscope,  the  blue  is  Hmited  to  the  nuclei  the  section  is  cleared  in 
xylol  and  mounted  in  damar. 

Borax  carmine  and  Lyons  blue  is  perhaps  the  best  general  stain  for 
embryos.  Dissolve  4  gr.  of  borax  in  100  cc.  of  hot  distilled  water.  When 
cool  stir  in  6  gr.  of  the  best  carmine  and  then  add  100  cc.  of  70  per  cent. 
alcohol.  After  24  hours,  filter.  The  Lyons  blue  may  be  used  in  i  per  cent, 
alcoholic  solution,  made  with  50  per  cent,  or  95  per  cent,  alcohol. 
Generally  it  is  desirable  to  dilute  it  somewhat  with  alcohol  before  using. 

Before  imbedding  the  tissue,  it  is  stained  in  borax  carmine  from  24  to 
48  hours,  larger  blocks  of  tissue  requiring  more  time  than  small  ones. 
After  being  placed  in  water  for  5  minutes  (a  step  which  some  omit),  the 
tissue  is  transferred  to  acid  alcohol  (0.5  cc.  of  hydrochloric  acid  in  ico  cc. 
of  70  per  cent,  alcohol).  In  this  the  excess  of  stain  comes  out  but  the  tissue 
acquires  a  deeper  color.  After  remaining  in  the  acid  alcohol  from  15 
minutes  to  an  hour  the  tissue  is  washed  thoroughly  in  70  per  cent,  alcohol 
and  is  imbedded  and  sectioned  in  paraffin  in  the  ordinary  way.  After  the 
sections  have  been  attached  to  the  slide  they  are  stained  in  Lyons  blue, 
rinsed  in  alcohol,  dehydrated,  cleared  and  mounted. 

Special  Stains. 
An  attempt  to  present  all  of  the  important  histological  stains  would 
exceed  the  desired  hmits  of  this  book.     The  four  modifications  of  Golgi's 


412  HISTOLOGY. 

method,  the  very  important  but  complex  Weigert  stain  for  myehn,  and  the 
iron  haematoxyhne  stain  for  cytological  details  are  omitted  with  many 
others.  Since  they  are  so  well  described  in  Mallory  and  Wright's  Tech- 
nique, which  the  medical  student  who  intends  to  understand  bacteriological 
and  histological  methods  should  possess,  it  seems  best  to  hmit  this  account 
to  the  stains  which  the  beginner  may  employ. 

Elastic  -fibers  are  stained  dark  purple  or  almost  black  with  Weigert's 
resorcin-fuchsin.  Other  parts  of  the  tissue  should  be  nearly  colorless. 
The  stain  is  prepared  by  heating  until  it  boils,  in  an  evaporating  dish,  2  gr. 
of  fuchsin  and  4  gr.  of  resorcin  in  200  cc.  of  water.  Then  25  cc.  of 
liquor  ferri  sesquichlorati  are  added  and  the  mixture  is  boiled  for  5  minutes. 
It  is  cooled  and  filtered  in  order  to  collect  the  precipitate.  The  dish  in 
which  the  boiling  took  place  is  dried,  together  with  whatever  precipitate 
remained  in  it,  and  after  the  precipitate  upon  the  filter  paper  is  also  dry  it  is 
placed  with  the  paper  in  the  dish.  200  cc.  of  95  per  cent,  alcohol  are  added 
and  boiled  to  dissolve  the  precipitate;  the  paper  is  removed.  When  the 
solution  has  cooled  it  is  again  filtered  to  collect  the  filtrate.  4  cc.  of  hy- 
drochloric acid,  and  enough  95  per  cent,  alcohol  to  make  up  200  cc.  of  stain, 
are  added. 

In  this  solution  paraffin  or  celloidin  sections  may  be  stained  from  20 
minutes  to  an  hour;  then  they  are  washed  in  alcohol,  dehydrated,  cleared, 
and  mounted.  If  the  stain  has  affected  other  parts  of  tbe  tissue  than  the 
elastic  fibers,  the  sections  should  be  washed  in  alcohol  containing  "a  few 
crystals  of  picric  acid,"  or  in  alcohol  containing  i  per  cent,  of  hydrochloric 
acid. 

White  fibers  of  connective  tissue  may  be  stained  by  Mallory's  aniline 
blue.  Fibrils  of  connective  and  reticular  tissue,  amyloid,  and  mucus 
stain  blue;  nuclei,  protoplasm,  muscle,  nerves  and  neurogha  fibres  stain 
red;  red  corpuscles  and  myehn  stain  yellow.  Paraffin  or  celloidin  sections 
of  material  fixed  in  Zenker's  fluid  are  stained  5  minutes  or  longer  in  a  -jV 
per  cent,  aqueous  solution  of  acid  fuchsin.  They  are  transferred  directly  to 
a  stain  consisting  of  0.5  gr.  of  aniline  blue  soluble  in  water,  and  2  gr.  of 
orange  G,  dissolved  in  a  100  cc.  of  a  i  per  cent,  aqueous  solution  of  phos- 
phomolybdic  acid.  In  this  they  remain  20  minutes  or  longer.  They 
are  washed  in  several  changes  of  95  per  cent,  alcohol,  cleared,  and  mounted. 

Fat  may  be  stained  red  in  frozen  sections  of  fresh  material  or  of  that 
hardened  in  formaline,  by  means  of  a  saturated  solution  of  Scharlach  R. 
in  70  per  cent,  alcohol  The  frozen  sections  are  transferred  from  water  to 
the  stain,  which  has  been  filtered  and  is  kept  tightly  stoppered,  since 
evaporation  of  the  alcohol  causes  a  precipitation  of  the  stain.  The  sections 
remain  in  the  stain  from  15  minutes  to  over  night;  then  they  are  washed  in 


SPECIAL     STAINS.  413 

water,  stained  with  haematoxyline  and  mounted  in  glycerine  which  clears 
them.  They  are  not  dehydrated  in  alcohol  since  strong  alcohol  dissolves 
the  fat  and  its  stain. 

Osmic  acid  in  i  per  cent,  aqueous  solution  stains  fat  in  fresh  tissues 
dark  brown  or  black;  myelin  responds  like  fat  both  to  osmic  acid  and 
Scharlach  R,  The  fat  is  blackened  in  tissues  preserved  in  a  mixture  of  2 
parts  of  Miiller's  fluid  (p.  404)  and  i  part  of  the  i  per  cent,  osmic  acid  solu- 
tion. Tissues  should  remain  in  it  for  about  a  week,  after  which  they  are 
transferred  to  dilute  alcohol  (50-70  per  cent.)  for  a  few  days.  They  may 
then  be  imbedded  in  paraffin  in  the  usual  way,  since  the  stained  fat  is 
rendered  insoluble  in  alcohol;  it  dssiolves  in  xylol  however,  so  that  the 
sections  should  be  cleared  in  chloroform  and  mounted  in  damar  dissolved  in 
chloroform. 

Blood  may  be  stained  for  the  study  of  leucocyte  granules  and  blood 
plates  with  Wright's  stain  which  should  be  prepared  as  follows:  After  0.5 
gr.  of  sodium  bicarbonate  has  been  completely  dissolved  in  100  cc.  of  dis- 
tilled water,  add  i  gr.  of  Griibler's  methylene  blue  (either  the  form  called 
BX,  Koch's,  or  Ehrhch's  rectified).  "The  mixture  is  next  to  be  steamed  in 
an  ordinary  steam  sterilizer  at  100°  C.  for  one  hour,  counting  the  time  after 
steam  is  up.  The  heating  should  not  be  done  in  a  pressure  sterilizer,  or 
in  a  water  bath,  or  in  any  other  way  than  as  stated."  After  the  steaming 
the  mixture  is  taken  from  the  sterilizer  and  allowed  to  cool,  the  flask  being 
placed  in  cold  water  if  desired.  When  cold  it  is  poured  into  a  large  dish  or 
flask.  To  ICO  cc.  of  the  mixture  add  about  500  cc.  of  a  -^q  per  cent,  solution 
of  Griibler's  yellowish  eosine  soluble  in  water.  The  amount  of  the  eosine 
solution  should  be  determined  by  the  appearance  of  the  mixture  which  it 
forms,  the  whole  being  stirred  if  in  a  dish,  or  shaken  if  in  a  flask,  while  the 
eosine  is  added.  The  color  changes  from  blue  to  purple,  and  a  yellowish 
metallic  scum  forms  on  the  surface,  "while  on  close  inspection  a  finely 
granular  black  precipitate  appears  in  suspension."  The  solution  is  then 
filtered  and  the  precipitate  is  allowed  to  become  perfectly  dry  upon  the 
filter  paper.  The  stain  is  made  by  dissolving  0.5  gr.  of  the  precipitate  in 
100  cc.  of  pure  methyl  alcohol.  The  stain  need  not  be  filtered,  and  like  the 
precipitate  it  keeps  indefinitely.  If  by  evaporation  of  the  alcohol  it  becomes 
too  concentrated,  as  is  shown  by  the  formation  of  precipitates  when  it  is  used, 
it  should  be  filtered  and  a  small  quantity  of  methyl  alcohol  added. 

Blood  is  obtained  usually  from  a  needle  puncture  in  the  lobule  of  the 
ear.  Two  cover  glasses,  perfectly  clean  and  dry,  should  be  at  hand.  When 
the  blood  is  flowing  freely,  the  center  of  one  of  the  covers  is  touched  to  a 
small  drop  as  it  emerges,  and  is  then  immediately  inverted  and  dropped 
upon  the  other  cover.     The  blood  should  spread  evenly  between  the  two 


414  HISTOLOGY. 

cover  glasses,  forming  a  film  which  cannot  be  too  thin.  The  covers  are 
then  drawn  rapidly  apart,  sliding  over  one  another,  and  the  blood  dries 
from  exposure  to  the  air.     It  remains  stainable  for  weeks. 

To  stain  the  blood  film,  the  cover  glass  may  be  held  in  the  forceps 
devised  for  this  purpose  (cover-glass  forceps),  with  the  film  uppermost. 
Stain  sufficient  to  cover  it  is  poured  upon  it,  and  after  one  minute  several 
drops  of  distilled  water  are  added  to  the  stain,  until  a  delicate  metallic  scum 
forms  upon  the  surface.  The  stain  should  not  be  so  diluted  as  to  become 
transparent.  After  two  or  three  minutes,  the  stain  is  washed  off.  The 
preparation  appears  blue.  Distilled  water  is  placed  upon  it  to  extract  the 
excess  of  stain  and  the  color  changes  to  orange,  or  pink  if  the  decolorization 
proceeds  further.  The  general  color  of  the  specimen  is  due  to  that  of  the 
the  red  corpuscles  which  at  first  are  blue.  When  they  have  become  orange 
or  pink  as  is  desired,  the  water  is  removed  by  applying  several  layers  of 
filter  paper,  and  the  preparation  is  mounted  in  damar.  The  process  of 
decolorizing  may  be  watched  through  the  microscope  by  placing  the  cover 
glass  (with  the  film  side  up)  on  a  shde.  Thicker  portions  of  the  film  which 
remain  blue  when  the  thinner  parts  are  orange,  should  be  disregarded. 
The  leucocytes  are  figured  on  page  147. 

Intercellular  cement  spaces  and  the  boundaries  of  endothelial  cells 
may  be  blackened  by  a  i  to  i  per  cent,  solution  of  silver  nitrate,  which  acts 
chiefly  upon  free  surfaces.  The  fresh  tissue  should  be  kept  flat,  the 
mesentery  for  example  being  tied  over  a  bottle  neck,  while  it  is  immersed  in 
the  solution  for  from  i  to  10  minutes.  Then  it  is  placed  in  distilled  water 
and  exposed  to  direct  sunlight.  As  soon  as  it  becomes  brown  (usually  in 
5  or  10  minutes)  it  is  washed  in  dilute  salt  solution  and  slowly  hardened  in 
graded  alcohols.  Larger  blood  vessels  may  be  injected  through  glass  tubes 
with  the  silver  solution,  and  after  sections  have  been  made  and  exposed  to 
the  light,  the  endothelial  cell  outhnes  become  dark. 

The  courses  0}  blood  and  lymphatic  vessels  and  of  ducts  are  studied 
by  means  of  injections.  Colored  fluids,  usually  such  as  harden  by  coohng  or 
otherwise,  are  forced  into  them  by  pressure  from  a  syringe.  The  syringe 
is  connected  by  a  short  rubber  tube  with  a  tapering  glass  tube  or  cannula; 
the  latter  is  inserted  into  the  vessel  which  is  then  tied  securely  around  it. 
Pressure  may  also  be  obtained  by  having  the  injection  mass  in  a  receptacle 
which  connects  with  the  cannula  by  a  long  flexible  tube;  pressure  is  in- 
creased by  elevating  the  receptacle.  The  organs  to  be  injected  must  be 
fresh;  they  may  be  left  within  the  body  or  removed  and  injected  separately. 
To  avoid  undue  distension  of  the  vessels  and  to  allow  the  injection  to  flow 
more  readily,  the  efferent  vessels  may  be  cut,  so  that  the  blood  escapes. 
Sometimes  the  vessels  are  washed  out  by  a  prehminary  injection  of  salt 


INJECTIONS.  415 

solution.  The  efferent  vessels  may  be  tied  to  cause  the  smaller  side  branches 
to  be  filled.  After  the  injection  has  been  finished,  the  tissues  may  be 
hardened  in  alcohol  or  Miiller's  fluid,  and  sectioned  in  the  usual  way; 
thick  sections  are  necessary  in  order  to  follow  the  course  of  the  vessels. 

Solutions  of  Berhn  blue  or  India  ink  are  the  simplest  injection  fluids. 
Carmine  may  be  prepared  by  dissolving  i  gr.  in  the  required  amount  of 
ammonia  and  adding  20  cc.  of  glycerine.  The  solution  is  completed  by 
adding  i  gr.  of  common  salt  dissolved  in  30  cc.  of  glycerine  (or  20  drops  of 
hydrochloric  acid  in  20  cc.  of  glycerine).  The  second  solution  is  to  neutral- 
ize the  first  solution,  since  the  ammoniacal  fluid  tends  to  spread  through  the 
vessel  walls. 

Gelatin  injection  masses  are  used  while  warm  and  fluid,  and  the  tissues 
which  receive  them  must  be  kept  warm  in  a  water  bath.  Clean  sheets  of 
the  best  French  gelatin  are  soaked  in  water  for  several  hours,  imtil  soft 
and  swollen.  Then  they  are  melted  over  a  water  bath  and  an  equal  quan- 
tity of  an  aqueous  solution  of  Berhn  blue,  saturated  or  dilute  as  desired,  is 
stirred  in.  The  mass  is  filtered  through  flannel  wrung  out  in  hot  water, 
and  is  injected  while  warm. 

A  carmine  mass  may  be  prepared  by  dissolving  from  2  to  4  grs.  of  the 
best  carmine  in  the  required  amount  of  ammonia.  The  solution  is  filtered 
and  stirred  into  filtered  melted  gelatin  prepared  as  already  described. 
The  amount  of  gelatin  may  be  from  10  to  50  grs.  Twenty-five  per  cent, 
acetic  acid  is  then  added  drop  by  drop,  until  the  mass  becomes  bright  red 
and  loses  its  ammoniacal  odor.  If  too  much  acetic  acid  is  added  a  pre- 
cipitate forms  and  the  mass  is  spoiled.  During  the  process  the  mixture  is 
kept  warm  over  a  water  bath  and  is  constantly  stirred.  It  is  filtered  through 
warm  flannel  and  may  be  used  at  once  or  allowed  to  cool  and  heated  when 
needed. 

Prepared  injection  masses  are  sold  by  Griibler. 

Many  ingenious  injection  methods  have  been  devised,  such  as  the  in- 
jection of  smaU  hving  pig  embryos  by  allowing  ink  to  enter  the  umbiHcal 
vein  and  be  distributed  through  the  body  by  the  heart's  action;  or  the 
injection  of  vessels  with  milk  and  staining  the  frozen  sections  with 
Scharlach  R. 

The  Microscope. 

It  is  unfortunate  that  the  price  of  a  microscope  is  prohibitive  to  many 
medical  students,  and  that  some  who  would  otherwise  purchase  instru- 
ments at  the  beginning  of  their  work,  wait  until  an  official  position  entitles 
them  to  a  discount.  The  price  of  microscopes  is  not  always  quite  as  high 
as  is  fisted,  and  sometimes  when  several  students  buy  microscopes  at  one 
time  they  may  secure  lower  rates  by  having  one  of  their  number  act  as  agent. 


4l6  HISTOLOGY. 

Within  the  past  ten  years  the  cost  of  a  good  instrument  has  been  so  reduced 


Eye-piece  (Ocular) 


Draw-tube 


.  Back  and  pinion  adjustment 


Micrometer  Screw 


that  an  increasing  proportion  of  students  can  enjoy  the  advantage  of  having 
a  microscope  of  their  own. 


MICROSCOPES.  417 

Microscopes  of  a  certain  grade  are  required,  and  if  they  cannot  be 
afforded,  no  instrument  should  be  bought.  The  necessary  equipment,  as 
shown  in  the  figure,  is  a  stand  with  fine  and  coarse  adjustment  ("microm- 
eter screw"  and  "rack  and  pinion")  and  a  large  square  stage.  The  more 
expensive  round  and  mechanical  stages  are  not  necessary'.  There  should 
be  an  Abbe  condenser  (with  iris  diaphragm),  a  triple  revolver,  a  high  and 
a  low  eye-piece  or  ocular,  and  the  follo^^■ing  objectives:  a  f  inch  and  a 
^  or  1^  inch,  which  must  be  parfocal,  together  with  a  -^  oil  immersion  for 
cytological  and  bacteriological  work,  and  a  2  inch  (very  low  power)  for 
embiyological  work.  The  ^2  oil  immersion  is  an  expensive  objective, 
and  its  purchase  may  be  postponed.  The  2  inch  is  a  cheap  objective  which 
is  very  useful  in  obtaining  a  view  of  an  entire  section,  and  for  embr}^ological 
reconstructions  it  is  essential.  The  price  of  such  an  outfit,  including  the 
oil  immersion  objective,  is  from  $70.00  to  $90.00. 

Satisfactory  microscopes  of  American  manufacture  are  made  by  the 
Bausch  &  Lomb  Company.  A  sample  submitted  by  the  Spencer  Lens 
Company  to  the  Harvard  Embryological  Laboratory  is  also  quite  satis- 
factor}' .  The  Leitz  microscopes,  made  in  Germany,  are  preferred  by  some 
to  the  American  instruments  just  described;  they  are  not  much  more  ex- 
pensive. All  agree  that  the  Zeiss  microscopes  (German)  are  the  best  (and 
most  expensive).  It  is  undoubtedly  true  that  any  of  these  instruments  will 
fill  the  requirements  of  medical  students  and  physicians.  If  the  micro- 
scope is  purchased  by  a  student  unfamilar  with  its  use,  it  is  well  to  have  the 
lenses  examined  by  a  disinterested  microscopist. 

For  a  description  of  the  nature  and  use  of  the  microscope,  the  student 
is  referred  to  the  9th  edition  of  "The  Microscope, "  by  Professor  S.  H.  Gage, 
(Comstock  Pub.  Co.,  Ithaca,  N.  Y.). 

For  the  sake  of  emphasis  it  may  be  said  that  the  microscopist  works 
with  his  right  hand  upon  the  fine  adjustment  and  his  left  hand  upon  the 
slide.  As  the  latter  is  moved  about,  bringing  different  iields  into  view, 
the  focussing  is  done  with  the  adjustment  and  not  with  the  eyes.  Both 
eyes  should  be  open  (as  will  be  natural  after  becoming  accustomed  to  the 
instrument).  Often  one  acquires  the  habit  of  using  only  the  right  or  the 
left  eye  for  microscopic  work,  but  it  is  better  to  learn  to  use  both. 

Always  examine  a  specimen  first  with  a  low  power  and  then  with  a 
high  power  objective.  In  focussing  the  microscope,  have  the  objective 
drawn  away  from  the  slide  and  focus  do\NTi.  This  should  be  done  cautious- 
ly, with  a  portion  of  the  specimen  actually  beneath  the  lens ;  if  there  is  only 
cover  glass  and  damar  there,  the  objective  wiU  probably  be  driven  down 
upon  the  sHde.  Unless  one  is  sure  that  stained  tissue  is  in  the  field,  the 
slide  should  be  moved  back  and  forth  as  the  objective  is  being  lowered. 
27 


4l8  HISTOLOGY. 

In  working  with  the  Abbe  condenser  the  flat  surface  of  the  mirror 
should  be  uppermost. 

The  objectives  must  never  be  scratched.  Lens  paper  or  fine  linen 
should  be  used  to  wipe  them.  If  they  are  soiled  with  damar  they  should 
be  wiped  with  a  cloth  moistened  with  xylol.  Since  the  lenses  are  mounted 
in  balsam,  xylol  must  be  applied  to  them  cautiously. 

In  lifting  the  microscope  it  should  never  be  taken  by  any  part  above 
the  stage;  the  pillar  should  be  grasped  below  the  stage. 

Drawings. 

Drawings  should  be  made  of  all  the  significant  structures  observed; 
the  structure  should  be  observed  however,  before  any  drawing  is  attempted. 
In  other  words  a  thorough  study  of  the  specimen  should  precede  the  draw- 
ing. The  nondescript  character  of  many  drawings  seems  due  to  the  fact 
that  the  student  had  nothing  definite  in  mind  to  portray.  It  is  true  never- 
theless that  the  repeated  observation  made  while  a  careful  drawing  is  in 
progress,  reveals  many  details  which  would  otherwise  be  overlooked. 

The  drawings  should  be  simple  but  exact,  made  and  shaded  with  a 
hard  (6  H)  lead  pencil  having  a  sharp  point.  They  should  not  be  encum- 
bered with  surrounding  circles.  The  parts  are  to  be  labelled  in  one's 
plainest  handwriting  (not  printing);  and  the  terms  should  be  explicit. 
A  line  proceeding  from  a  mass  of  chromatin  within  a  cell  nucleus  ought  not 
to  be  labelled  either  cell  or  nucleus  but  chromatin.  Some  knowledge  of 
drawing  is  very  desirable  although  perspective  is  scarcely  involved  in  histo- 
logical work.  The  lightly  colored  structures  should  be  made  lighter  and 
the  dark  ones  darker  than  they  appear,  to  preserve  the  contrasts  of  the 
stains.  The  fines  should  be  few  and  made  with  assurance, — not  pieced 
out  as  if  one  were  feefing  his  way.  Every  line  should  correspond  with  some 
structure;  if  a  cell  has  no  wall,  the  even  or  granular  shading  representing 
its  protoplasm  should  end  abruptly,  but  without  a  bounding  line. 

Reconstructions. 
.  There  is  an  important  arrangement  of  mirrors  (Abbe's  camera  lucida) 
for  drawing  the  outlines  of  sections.  It  is  attached  to  the  microscope  so 
that  the  image  of  the  section  beneath  the  objective  appears  spread  upon  the 
drawing  paper.  The  paper  is  on  the  table  beside  the  base  of  the  microscope. 
On  looking  through  the  camera  into  the  microscope  one  can  see  the  pencil 
point,  as  it  is  made  to  trace  the  outfine  on  the  paper.  In  this  way  a  suc- 
cession of  serial  sections  may  be  drawTi  with  uniform  magnification.  The 
magnification  is  determined  by  substituting  a  stage  micrometer  for  the  slide 
of  sections.     The  micrometer  is  a  sfide  upon  which  i  mm.,  with  subdivisions 


RECONSTRUCTIONS.  419 

into  twentieths  or  hundredths,  has  been  marked  off  by  scratches  in  the  glass; 
the  subdivisions  may  be  drawn  with  the  camera  under  the  same  conditions  as 
the  sections,  and  the  enlargement  of  the  subdivisions  may  then  be  measured. 

From  the  camera-drawings  of  serial  sections,  wax  reconstructions  of 
adult  glands  or  embryonic  organs  may  be  made.  If  the  sections  are  lo  ij. 
thick  and  alternate  sections  have  been  drawTi,  magnified  50  diameters, 
then  on  the  scale  of  the  drawings  these  alternate  sections  are  i  mm.  apart. 
Wax  plates  imm.  thick  are  therefore  to  be  made,  either  by  rolhng  the  wax, 
or  by  spreading  a  weighed  amount  of  melted  wax  in  a  pan  of  hot  water. 
It  floats  and  spreads  in  an  even  layer,  sohdifying  as  the  water  cools.  The 
outlines  of  the  drawings  are  then  indented  upon  the  w^ax  plates,  and  the 
desired  portions  are  cut  out  and  piled  up  to  make  the  model.  In  this  way 
reconstructions  like  those  of  the  ear  (p.  380)  may  be  made.  The  details 
of  the  process  should  be  learned  from  demonstrations  in  the  laboratory. 

Graphic  reconstructions  are  usually  side  views  of  structures,  made  from 
measurements  of  their  transverse  sections.  Fig.  161,  p.  138,  is  from  such  a  re- 
construction. A  camera  drawing  of  the  side  of  an  embryo  for  other  struc- 
ture) is  made  before  it  is  sectioned.  The  outline  of  this  drawing  is  en- 
larged, and  parallel  hnes  equally  spaced  are  ruled  across  it,  corresponding 
in  number  and  direction  with  the  sections  into  which  it  was  cut.  Often 
only  every  other  section  or  every  fourth  section  is  used  for  the  reconstruc- 
tion, and  the  number  of  hnes  to  be  ruled  across  the  drawing  is  correspond- 
ingly reduced.  Camera  drawings  of  a  lateral  half  of  ever}'  section  used  in 
the  reconstruction  are  made,  and  across  each  drawing  two  hnes  are  ruled. 
The  first  follows  the  median  plane  of  the  body;  and  the  second  is  at  right 
angles  with  it,  being  drawTi  so  as  to  touch  the  dorsal  or  ventral  surface  of 
some  structure  to  be  included  in  the  reconstruction.  Provided  that  the 
camera  drawings  and  side  view  have  been  enlarged  to  the  same  extent,  the 
perpendicular  distance  from  the  middle  of  the  back  to  the  junction  of  the 
two  hnes  is  marked  off  on  the  side  view,  on  the  hne  corresponding  with  the 
section  in  question.  The  perpendicular  distances  from  the  second  hne  to 
the  dorsal  and  to  the  ventral  surfaces  of  all  structures  to  be  reconstructed 
are  also  marked  off"  upon  the  line  in  the  side  view.  The  same  is  done  in  the 
following  section,  and  the  points  belonging  with  a  given  structure  are  con- 
nected from  section  to  section.  Thus  the  outlines  of  the  organs  are  pro- 
jected upon  the  median  plane;  two  dimensions  are  accurately  sho\\'n  but 
the  third  is  lost. 

Often  it  is  undesirable  to  attempt  to  make  the  magnification  of  the 
sections  and  of  the  side  view  identical;  the  measurements  may  be  en- 
larged or  reduced  as  they  are  transferred  for  plotting,  by  means  of  the 
draughtsman's    proportional  dividers, — an    indispensable    instrument    for 


420  HISTOLOGY. 

this  method  of  reconstruction.  Tlie  corrections  for  unequal  shrinkage  of 
the  sections  in  paraffin  and  other  details  can  best  be  explained  in  the  labora- 
tory with  the  drawings  at  hand. 


INDEX. 


Abducens  nerve,  96,  336 
Absorption,  intestinal,  209 
Accessory  lachrymal  glands,  375 

nerve,  96,  335 

parotid  glands,  187 

thyreoid  glands,  173 
Acervulus  cerebri,  350 

Acetic  acid,  action  on  connective  tissue,  401 
Acidophiles  {eosinophiles),  148 
Acinus,  36 

Acoustic  nerve,  97,  335,  389 
Adamantoblasts,  70 
Adelomorphous  cells  {chief  cells),  200 
Adenoid  tissue  {lymphoid  tissue),  39,  154 
Adipose  tissue,  41 

Adrenal  glands  {suprarenal  glands),  331 
Aggregate  nodules,  156,  214 
Agminated  nodules  {aggregate  nodules),    156 

214 
Albumen,  for  attaching  sections  to  slides,  408 
Alcohol,  for  dehydrating  tissues,  409 

for  fixation,  405 

for  hardening  tissues,  403 
Allantois,  193,  310 
Alveolar  duc^s,  240 

periosteum,  77 

sacs,  240 
Alveoli  of  the  lungs,  236,  240 
Alveolus,  36 

Amakrine  cells,  359,  360 
Ameloblasts  {adamantoblasts) ,  70 
Amitosis,  14 

Ammon's  horn  {hippocampus) ,  349 
Amnion,  300,  303 
Amniotic  fluid,  244,  302 

villi,  311 
Amoeboid  motion,  8 
Amphiaster,  13 
Amphipyrenin,  5 
Ampulla,  of  the  ductus  deferens,  278 

of  the  semicircular  ducts,  379 

of  the  uterine  tubes,  294 
Ampullary  nerves,  389 
Anal  plate,  21 
Anaphase,  12 
Angioblast,  23 
Anisotropic  substance,  82 
Annuli  fibrosi,  134 
Anterior  neuropore,  23 
Anus,  193,  217 
Aorta,  130 

Appendices  epiploicae,  217 
Appendix  epididymidis,  280 


Appendix  testis,  So 

vesiculosa,  286 

vermiform  {processus  vermiformis),   215 
Aquaeductus  cerebri,  334 

cochleae,  391 

vestibuli,  390 
x\queous  humor,  355 
Arachnoid,  351 

granulations,  351 
Archoplasm,  6 
Areola,  330 
Areolar  glands,  330 

tissue,  41 
Arrector  pili,  317 

Arteria  centralis  retinae,  353,  373 
Arteries,  127 
Arterioles,  127 
Articular  cartilage,  64 

corpuscles,  105 
Astrosphere,  9 
Atretic  follicles,  292 
Atria,  of  the  heart,  133 

of  the  lung,  241 
Auditory  groove,  166 

tube,  116,  382,  393 

vesicle,  353,  378 
Auerbach's  plexus  {myenteric  plexus),  95,  215 
Auricle,  382 

of  the  heart  {atrium),  133 
Axial  filament,  271 
Axis  cylinder,  98 
Axolemma,  99 
Axon  {neuraxon),  93,  94 


B. 

Bartholin's  ducts  {sublingual),  190 

glands  {major  vestibular),  311 
Basal  body,  31 
Basement  membrane,  30 
Basket  cells,  of  cerebellum,  344 

of  mammary  gland,  329 

of  pancreas,  233 
Basophile  cells,  47,  148 
Bergamot  oil  for  clearing  sections,  409 
Berlin  blue  for  injections,  415 
Bertini,  columns  of  {renal  columns),  251 
Bile,  226 
Bile  capillaries,  226 

ducts,  219,  229 
Bipolar  cells,  109 
Bladder,  261 

development,  193 
Blast,  54 


421 


4^ 


INDEX. 


Blastodermic  vesicle,  iS,  300 
Blood,  140 

arteries,  127 

capillaries,  126,  127 

cni'stals,  144 

destroying  organs,  152 

development  of  cardinal  veins,  220,  246 
of  pulmonary  veins,  235 
of  umbilical  veins,  220,  316 
of  vitelline  veins,  24,  220 
of  veins  of  liver,  220,  247 

forming  organs,  152 

heart,  133 

injections  of,  414 

islands,  23 

pigments,  46 

plasma,  151 

plates,  150 

red  corpuscles,  140 

sinus,  159 

sinusoids,  125 

stains  for,  413 

veins,  131 

vessels,  124 

white  corpuscles,  145 
Bone,  53 

blood  vessels  of,  59 

cartilage  replaced  by  bone,  61 

cells,  55 

compact,  57 

corpuscles,  55 

decalcification  of,  405 

development  of,  54 

lamellar,  57 

marrow,  152 

function  of,  153 
red,  154 
yellow,  154 

membrane,  61 

priman.-,  61 

secondar}-,  61 

spong}',  57 
Borax  carmine,  411 
Border  fibrils,  78 
Bowman's  glands  (oljactory  glands),  398 

capsule  (capsule  of  the  renal  glomerulus) , 
254 

membrane  (anterior  basal  membrane  of 
cornea),  369 
Brachium  conjunctivum,  336 

pontis,  336 
Brain,  91,  334-352 

cerebellum,  342 

development  of,  334 

hemispheres,  345 

hypophysis,  349 

medulla  oblongata,  339 

meninges,  351 

pineal  body,  350 

pons,  341 
Branchial  arches,  176 

clefts,  165 
Bridges,  intercellular,  30 
Bronchi,  239 
Bronchial  arteries,  235 

veins,  235 


Bronchioles,  239 

respirator}-,  240 
Brunner's  glands  {duodenal  glands),  204 
Bulbous  corpuscles,  106 
Bulbourethral  glands,  282 
Bulbus  urethrae,  283 

vestibuli,  311 
Bursae,  50 

C. 

Caecum,  216 

cupulare,  380 

vestibulare,  380 
Calcification  of  dentine,  74 
Calcified  cartilage,  52 
Calyces  of  kidney,  249 
Camera  lucida,  418 
Canaliculi,  of  bones,  55 

of  the  cornea,  82 
Capillaries,  blood,  127 

secretory,  36 
Capillar}-  circulation,  125 
Capsule,  of  cartilage  cells,  50 

of  Glisson  {capsule  oj  liver),  220 

of  kidney,  252 

of  lens,  354,  365 

of  liver,  220 

of  Tenon  (interfascial  space),  374 
Cardiac  ganglion,  94 

glands,  of  stomach,  200 

oesophageal,  197 

muscle,  80 

development,  80 

fibrils,  81 

intercalated  discs,  83 

nuclei,  84 

striations,  82 

veins,  220,  246 
Carmine,  for  injections,  415 
Carotid  gland  {glomus  caroticum).  176 
Cartilage,  50 

articular,  64 

elastic,  52 

epiphyseal,  64 

fibrous,  53 

hyaline,  52 
Caruncula  lacrimalis,  377 
Cell,  I 

amoeboid  motion  of,  8 

differentiation,  15 

direct  division,  14 

form,  7 

formation,  9 

indirect  division,  9 

size  of,  7 

vital  phenomena  of,  7 

wall,  6 
Celloidin,  imbedding  in,  406 
Cells,  amakrine,  359,  360 

basket,  233,  329,  344 

basophile,  47 

centroacinal,  233 

chromaffine,  114 

chief,  200 

commissural,  91 

decidual,  304 


IXDEX, 


423 


Cells,  Deiters'  (sustentacular  cells  oj  cochlea), 
388 

egg,  288 

eosinophilic,  47,  148 

ependymal,  115 

epithelial,  27 

fat,  43 

fiber-producing,  46 

follicular,  268,  289 

giant,  152 

glia,  lis 

goblet,  34 

Henson's,  389 

lutein,  291 

mast,  47,  148 

mucous  gland,  185 

neuroglia,  115,  349 

of  Claudius,  389 

of    Kupffer    (stellate    cells    of    hepatic 
sinusoids),  228 

of  Paneth,  207 

of  Purkinje,  344 

of  Retzius,  346 

of  Sertoli  (sustentacular  of  testis),  268 

olfactory,  397 

parietal,  200 

pignaented,  45 

plasma,  47 

polarity  of,  33 

pohnnorphonuclear,  146,  148 

pjTamidal,  346 

resting  wandering,  47 

serous  gland,  185 

sexual,  267,  288 

squamous,  29 

supporting,  179 

sustentacxilar,  267,  384,  388,  397 

tactile,  103 

taste,  181 

vegetative  ( siistentacidar)  of  the  testis,  268 

visual,  357 
Cellulae  pneumaticae,  393 
Cement  substance,  16 
Central  nervous  system,  91 
Centroacinal  cells,  233 
Centriole,  6 
Centrosome,  5 
Cerebellum,  336,  342 

development  of,  335 
Cerebral  hemispheres,  345 

ner\'es,  95 

of  medulla  and  pons,  336,  341 
Cerebrum  (includes  fore-brain  and  mid-brain) , 

334  . 
Ceruminous  glands,  313,  394 
Cervical  glands  of  the  uterus,  297 
Chief  cells,  200 
Choriocapillaris,  367 
Chorioid  coat  of  eye,  355,  366 

plexuses,  352 
Chorion,  300 

frondosum,  301 

laeve,  301,  303 
Chorionic  \alli,  300 
Chromaf&ne  cells,  114 

of  suprarenal  glands,  331,  ^^2 


Chromatic  bodies,  120 
Chromatin,  5 
Chromium,  14 
Chromosomes,  10 
Chyle,  209 
Chyme,  208 
Cilia,  30 

(eyelashes),  375 
Ciliar>'  body,  355,  367 

glands,  375 

muscle,  367 

nerves,  374 

processes,  367 
Circtimanal  glands,  326 

Circumvallate  papillae  {vallate  papillae),  177 
Cistema  chyli,  138 

Clarke,  column  of  (dorsal  nucleus),  117 
Claudius,  cells  of,  389 
Clasm.atoc}'tes,  47 
Clearing  sections,  409 
Clitoris,  287,  311 
Cloaca,  193 

Coccj-geal  gland  (glomus  coccygeum),  114,  176 
Cochlea,  385 

scala  tjtnpani,  385 

scala  vestibulae,  385 
Cochlear  artery^  390 

duct,  380 

nerve,  389 
Cohnheim's  areas,  87 
Coelom,  20 

Coil  glands  {sweat  glands),  325 
CoUagen,  41 

Collateral  nerve  fibers,  93 
Collecting  tubules  of  kidney,  249,  257 
Colloid,  173 
Colon,  216 
Colostrum,  328 
Columnar  epitheliima,  27 
Columns  of  spinal  cord,  115 
Commissural  cells,  91 

fibers,  91 
Commissure  of  spinal  cord,  115,  117 
Common  bile  duct,  219 
Conchae,  395 
Cone  cells,  357 
Conical  papillae,  117 
Conjunctiva  bulbi,  356,  377 

comeae,  377 

palpebrarum,  356,  375 

sclerae,  377 
Connective  tissue,  40 

cells,  43 

fibers,  41 

intercellular  spaces,  47 

stains,  43,  412 
Contour  lines,  72 
Convoluted  tubules,  of  kidney,  252 

of  testis,  273 
Coritim,  212,  213 
Cornea,  369 

development  of,  355 
Corona  radiata,  290 
Coronar\- sinus,  134 
Corpora  cavernosa  fin  female),  311 
penis,  284 


424 


INDEX. 


Corpora  quadrigemina,  336 
Corpus  albicans,  291 

callosum,  338 

cavernosum  urethrae,  286 

luteum,  290 

spongiosum,  262 

striatum,  338 
Corpuscles,  articular,  105 

bone,  55 

bulbous  (of  Krause),  106 

colostrum,  329 

cylindrical,  106 

genital,  106 

Golgi-Mazzoni,  243 

Hassall's,  172 

lamellar  (Pacinian),  107 

Malpighian,  253 

nerve,  105 

red  blood,  140 

renal,  253 

tactile  (of  Meissner),  105 

thymic,  172 

white  blood,  145 
Corpuscula  amylacea,  351 
Cortex,  156 

Corti,  organ  of  {spiral  organ),  387 
Cotyledons  of  placenta,  307 
Cowper's  glands  {bulbourethral  glands),  282 
Cranial  nerves  {cerebral  nerves),  95,  341 
Crescents  of  serous  cells,  186 
Cristae,  380 
Crusta,  30 

Crypts  of  Lieberkiihn  {intestinal  glands),  203 
Cumulus  oophorus,  290 
Cuboidal  epithelium,  27 
Cuticula,  6 

dentis,  73 
Cuticular  border,  30 
Cutis  {skin),  312 
Cuvier,  duct  of,  247 
Cylindrical  corpuscles,  106 
Cystic  duct,  219 
Cytomorphosis,  15 
Cytoplasm,  2 


Damar,  409 
Decalcification,  405 
Decidua  basalis,  301 

capsularis,3oi 

reflexa  {capstdaris),  301 

serotina  {basalts),  301 

vera,  301,  303 
Decidual  cells,  304 

membranes,  300 

relation  to  uterus,  301 
Decussation  of  the  lemnisci  [sensory],  340 

of  the  pyramids  [motor],  339 
Dehydration  of  sections,  409 
Deiters'  cells  {sustentacular  oj  cochlea),  388 
Delafield's  hematoxyline,  410 
Demilune,  186 
Dendrite,  93,  94 
Dental  canaliculi,  73 

cavity,  67 

fibers,  73 


Dental  groove,  68 

papilla,  68,  73 

pulp,  74 

ridge,  68 

sac,  76 
Dentine,  73 

calcification  of,  74 
Dermatome,  86 
Dermis  {corium),  312,  313 
Descemet's  membrane  {posterior  basal  mem- 
brane of  cornea),  371 
Diapedesis,  141 
Diaphragm,  219 
Diaphysis,  64 
Diarthrosis,  64 
Diencephalon,  335,  338 
Digestive  tube,  193 

development,  193 
Dilatator  muscle  of  pupil,  369 
Diplosome,  6 
Direct  cell  division,  14 
Dispireme,  12 

Diverticulum  of  the  intestine,  218 
Division  of  cells,  direct,  14 

indirect,  9 
Drawing  of  specimens,  418 
Ducts,  36 

Bartholin's  {sublingual),  190 

cochlear,  380 

common  bile,  219,  229 

cystic,  219,  229 

ejaculatory,  279 

endolymphatic,  370 

Gaertner's,  287 

intercalated,  96 

Miillerian,  263,  285 

of  Cuvier,  247 

of  Santorini  {accessory  pancreatic),  230 

of  Wirsung  {pancreatic),  230 

perilymphatic,  392 

semicircular,  379,  384 

Stenson's  {parotid  duct),  187 

utriculosaccular,  380 

Wharton's  {submaxillary  duct),  191 

Wolffian,  245,  285 
Ductulus  efferens,  266,  276 
Ductus  aberrantes,  265 

arteriosus,  234 

cochleae,  380,  385 

deferens,  265,  278 

epididymidis,  265,  277 

reuniens,  380 

venosus,  222 
Duodenum,  203 
Dura  mater  cerebralis,  351 

spinalis,  351 
Dvaster,  12 


E. 


Ear,  378-394 

development,  778 
external,  393 
internal,  384-392 
middle,  392 
nerves,  389 
vessels,  389 


INDEX. 


425 


Ectoderm,  18 

Efferent  ducts  of  testis,  266,  276 

Egg  cells,  2S8 

Ejaculatory  ducts,  279 

Elastic  fibers,  40,  41 

stain  for,  412 
Elastin,  42 

Embrj'os,  preservation  of,  404 
Enamel,  69 

development,  68 

prisms,  70 
Enamel  cells,  69 

organs,  68,  69 

pulp,  69 
End  bulb  of  Krause  {cylindrical  end  bulbs), 

106 
End  organs  of  Ruffini  {terminal  cylinder s) ,  106 
Endocardium,  126 
Endochondral  bone,  63 
Endolymph,  380 
Endolymphatic  duct,  379 

sac,  379 
Endoneurium,  loi 
Endoplasm,  2 
Endosteum,  57 
Endothelium,  25,  27 
Entoderm,  18 
Entodermal  tract,  165 
Eosine,  410 
Eosinophiles,  47,  148 
Ependyma,  115,  117,  122 
Epicardium,  126 
Epidermis,  312,  315 
Epididymis,  276 
Epithelia,  26-34 

layers,  28 

origin,  27 

shape  of  cells,  27 
Epithelioid  glands,  35 
Epithelium,  24 

basement  membrane,  30 

bridges,  30 

cilia,  30 

crusta,  30 

cuticular  border,  30 

differentiation  of  cells,  29 

false,  27 

glands,  35 

goblet  cells,  34 

membrana  propria,  30 

neuro-,  31 

pseudostratified,  28 

simple,  27 

secretory  processes,  32 

stratified,  29 

terminal  bars,  29 
Epitrichium,  312 
Eponychium,  316 
Epoophoron,  286,  294 
Erectile  tissue,  284 
Ergastoplasm,  33 
Erythroblast,  141,  153 
Erythrocytes,  140 

Eustachian  tube  {:i:ditory  tube),  382 
Excretions,  32 
Axoplasm,  2 


External  auditory  meatus,  382,394 
External  ear,  393 
Eye,  353-378 

blood  vessels,  371 

chambers,  354,  373 

development,  353 

lachrymal  glands,  377 

lens,  365 

nerves,  374 

optic  nerve,  364 

retina,  357 

spaces,  373 

tunica  fibrosa,  369 

tunica  vasculosa,  366 

vitreous  body,  366 
Eyelids,  375 

F. 

Facial  nerve,  96,  336 
Falciform  ligament,  279 
Fallopian  tubes  {uterine  tubes),  286 
Fasciculi,  cerebrospinalis,  121,  239 

cuneate,  122,  340 

gracile,  122,  340 

lateral  cerebrospinal,  122,  339 

superficial,  ventro-lateral,  122 

ventral  cerebrospinal,  122,  339 
Fat  cells,  43 

crystals,  44 

pigments,  46 

stains  for,  412 

tissue,  41 
Female  genital  organs,  285 
Fenestra  cochleae,  383 

vestibuli,  383 
Fenestrated  membrane,  42 
Ferrein,  pyramids  of  {pars  radiata),  251 
Fertilization,  293 
Fiber  cells  of  ear,  384 

of  Retzius,  346 

layer  of  Henle,  358 

tracts,  121 
Fibers,  elastic,  41 

of  Miiller  {radial  fibers  of  the  retina),  358 

of  Sharpey,  56,  76 

muscle,  79 

nerve,  90,  97 

white,  41 
Fibrin,  150 
Fibroblasts,  46 
Fibrocartilage,  53 
Fibroglia,  43 
Filar  mass,  3 
Filiform  papillae,  177 
Fillets  {lemnisci),  340 
Fimbria  ovarica,  287 
Fixation  of  tissues,  402 
Flagellum,  31 
Foliate  papillae,  177 
Follicle,  37 
Follicles,  atretic,  292 

formation  in  ovary,  288 

Graafian  {vesicular  follicles),  289 

lymph  of  ovary  (see  lytnph  nodules),  289 
Follicular  cells,  268,  289 


426 


INDEX. 


Fontana,  spaces  of  {spaces  oj  the  angle  oj  the 

Formaline,  404 
Foramen  caecum,  166 

epiploicum,  221 

of  Winslow  {epiploicum),  221 
Fore-brain,  334 
Fossa  of  Rosenmiiller,  167 
Fovea  centralis,  362 
Fresh  tissues,  examination  of,  400 
Freezing  tissues,  402 
Fundamental  tissues,  24 
Fundus  glands,  200 
Fungiform  papillae,  177 
Funiculi  of  spinal  cord,  116,  122 
Fuscin,  357 


G. 

Galea  capitis,  271 
Gall  bladder,  219,  229 
Ganglia,  91,  109 

cardiac,  94,  137 
ciliary,  97 
coeliac,  95 
of  Wrisberg,  137 
otic,  97 
semilunar,  95 
sphenopalatine,  97 
spinal,  92,  no 
submaxillary,  97 
sympathetic,  112 
Ganglion  cells,  109 
bipolar,  log 
multipolar,  109 
unipolar,  109 
Gastric  glands,  200 
Gelatin,  41 

injection  masses,  415 
Gelatinous  substance  of  spinal  cord,  \il 
Genital  corpuscles,  106 
folds,  266 
organs  (female),  285 

decidual  membranes,  300,  303 

development,  285 

epoophoron,  294 

external,  311 

ovary,  288 

placenta,  305 

umbilical  cord,  308 

uterine  tubes,  294 

uterus,  296 

vagina,  311 
organs  (male),  263 

appendices,  280 

development,  264 

ductus  deferens,  278 

ejaculatory  ducts,  279 

epididymis,  276 

paradidymis,  280 

penis,  266,  281 

prostate,  280 

scrotum,  266 

seminal  vesicles,  279 

testis,  267 

urethra,  266,  281 


Genital  papilla,  193,  266 

ridge,  264 
Germ  layers,  20 

origin  of  tissues  from,  26 
Germinal  epithelium  of  ovary,  288 
Giant  cells,  152 
GUI  clefts,  165 
Glands,  32 

alveolo-tubular,  36 

anterior  lingual,  190 

areolar,  330 

Bartholin's  {major  vestibular) , 

biliary,  229 

Bowman's  {olfactory),  398 

bronchial,  239 

buccal,  190 

bulbourethral,  282 

cardiac,  200 

ceruminous,  313,  394 

cervical,  of  uterus,  297 

ciliary,  375 

circumanal,  217 

classification,  34,  37 

compound,  34 

Cowper's  {bulbourethral),  282 

cytogenic,  34 

duodenal,  204 

ducts  of,  36 

epithelial,  35 

epithelioid,  35 

end  pieces  of,  37 

fundus,  200 

gastric,  200 

genital,  264 

intestinal,  203 

labial,  190 

lachrymal,  377 

lingual,  182,  189 

lumen  of,  36 

lymph,  154 

mammary,  313,  328 

Meibomian  {tarsal),  375 

mixed,  186,  190 

molar,  190 

mucous,  32,  189 

mucous  bile,  229 

oesophageal,  197 
cardiac,  197 

of  Brunner  {duodenal),  204 

of  Littre  {unethral),  282 

of  Moll  {ciliary),  375 

of  Montgomery  {areolar),  330 

of  the  oral  cavity,  185 

olfactory,  398 

palatine,  189 

peptic,  200 

praeputial,  324 

pyloric,  200 

sebaceous,  324 

secretory  capillaries  of,  36 

serous,  32,  187 

simple,  35 

sublingual,  190 

submaxillary,  191 

sudoriparous,  325 

sweat,  325 


INDEX. 


427 


Glands,  tarsal,  375 

tubulo-acinar,  36 

Tyson's,  324 

unicellular,  35 

vestibular,  311 

von     Ebner's     {serous     glands     of     the 
tongue),  187 

urethral,  262,  282 
Glans  penis,  260 
Glia  cells,  115 

Glisson's  capsule  {capsule  of  the  liver),  220 
Glomerulus,  126 

of  kidney,  252 

of  mesonephros,  245 
Glomus  caroticum,  114,  167,  176 

coccygeum,  114,  176 
Glossopharyngeal  nerve,  96,  335 

nuclei  of,  341 
Glycogen,  51 
Goblet  cells,  34 
Golgi-Mazzoni  corpuscles,  243 
Gowers'  bundle,  342 
Graafian  follicles,  289 
Grey  substance  of  spinal  cord,  119 

types  of  cells  in,  120 
Ground  substance,  16 
Gubernaculum  testis,  267 
Gustatory  organ  {taste  buds),  179 
Gyrus  hippocampi,  349 


H. 

Haematoidin,  144 
Haematoxyline,  401,  410 
Haemin,  144 
Haemoglobin,  140 

derivatives,  46 
Haemolymph  glands,  159 
Haemosiderin,  144 
Hair,  317 

bulb,  317 

connective  tissue  sheaths,  319 

epithelial  sheaths,  320 

lanugo,  319 

papilla,  317 

root,  317 

shaft,  322 

shedding  of,  322 
Hair  cells,  384,  387 
Hardening  tissues,  402 

Hassall's  corpuscles  {thymic  corpuscles),  172 
Haustrum,  217 
Haversian  canal,  56 
Heart,  133 

development  of,  133 

epicardium,  134,  135,  136 

myocardium,  134,  136 

muscles,  135,  136 

nerves,  137 

pericardium,  135 
_  valves,  134,  135 
Heidenhain,  ground  membrane  of,  82 
Hemispheres,  338,  345 
Henle's  fiber  layer,  358 

layer,  320 

loop,  252 


Henle's  sheath,  loi 
Hanson's  cells,  389 
Hepatic  arteries,  125,  222 

cells,  224 

duct,  219,  222 

trabeculae,  218 
Hind-brain,  334 
Hippocampus,  349 
Horns  of  spinal  cord  {columns),  115 
Howship's  lacunae,  57 
Huxley's  layer,  320 
Hyaline  cartilage,  52 
Hyaloid  artery,  355 

canal,  354 

membrane,  366 
Hyaloplasm,  3 
Hydatid  of  Morgagni,  280 

sessile,  280 

stalked,  280 
Hymen,  285 
Hypoglossal  nerve,  96,  335 

nucleus  of,  341 
Hypophysis,  349,  389 


Idiozome,  6,  271 

Ileum,  205 

Incisures,  99 

Inclusions,  4 

Incus,  382 

India  ink  for  injections,  415 

Infundibulum  of  the  fore-brain,  339 

of  the  lungs,  241 

of  the  uterine  tubes,  294 
Injections  of  vessels  and  ducts,  414 
Intercalated  discs,  83 
Intercellular  bridges,  30 

secretory  capillaries,  36 

substance,  16 
Interfascial  space,  374 
Intermedins  nerve,  96,  .335 
Internal  auditory  meatus,  389 

secretions,  35 
Interstitial  cells  of  testis,  275 
Intestinal  absorption,  209 

glands,  203 

villi,  204 
Intestine,  large,  215 

small,  203 
Involuntary  muscle,  77 
cardiac,  80 
smooth,  77 
Iris,  368 

development,  355 
Islands  of  Langerhans,  231 
Isolation  of  tissues,  401 
Isotropic  substance,  82 
Isthmus,  335,  336 

J. 

Jacobson's  organ  {vomero-nasal  organ),  397 
Jejunum,  207 
Joint  cavity,  65 
Joints,  64 


428 


INDEX. 


K. 

Karyokinesis,  9 
Karyoplasm,  2 
Keratohyalin,  184 
Kidney,  249-259 

blood  vessels,  257 

connective  tissue,  256 

convolute  part  of  the  cortex,  253 

cortex,  250 

development,  249 

labyrinth  {pars  convoluta),  253 

lobes,  257 

lymphatic  vessels,  258 

medulla,  250 

medullary  rays  {pars  radiata),  251 

nerves,  259 

pyramids,  250 

radiate  part  of  cortex,  251 

renal  columns,  251 

renal  tubules,  250-256 

structural  units,  257 
Krause's  corpuscles  {bulbous  corpuscles),  106 

cylindrical    end   bulbs    {cylindrical   cor- 
puscles), 106 

ground  membrane,  82 
Kupffer's  stellate  endothelial  cells,  228 


Labia  majora,  287 

minora,  287 
Labium  tympanicum,  387 

vestibulare,  387 
Labyrinth,  bony,  of  the  ear,  382 

membranous,  of  the  ear,  382 

of  the  kidney  {pars  convoluta),  253 
Lachrymal  glands,  377 
accessory,  375 

ducts,  378 

sac,  378 
Lacteals,  151,  213 
Lactiferous  sinus,  330 
Lacunae,  of  bone,  55 

of  cartilage,  50 

Howship's,  57 

urethral,  282 
Lamellae  of  bone,  56 
Lamellar  corpuscles,  107 
Lamina  cribrosa,  365 

chorio-capillaris,  367 

fusca,  366 

spiralis,  385 

suprachorioidea,  366 
Langerhans,   cells  of   (deeper  layer    of    the 
chorionic  epithelium),  305 

islands  of,  231 
Lantermann's  segments,  99 
Lanugo,  319 
Large  intestine,  215 

development,  195 

nerves,  217 

vessels,  217 
Larynx,  237 
Lemniscus,  340 
Lens,  365 

development,  353,  354 


Lentic  vesicle,  353 
Lesser  omentum,  219 
Leucocytes,  140,  145 

granules  of,  147 

varieties  of,  149 
Lieberkiihn,  crypts  oi  {intestinal  glands),  203 
Ligament,  65 

pectinate,  371 

suspensory,  of  lens  {zona  ciliaris),  364 
liver  {falciform  ligament),  219 
Limbus  spiralis,  386 
Lines  of  Retzius,  72 
Lingual  glands,  182,  189 

papillae,  177 

tonsil,  169,  184 
Linin,  5 

Lipochromes,  46 
Lips,  184 
Liquor  amnii,  244,  302 

cerebrospinalis,  352 

folliculi,  290 
Littre,  glands  of  {urethral  glands),  282 
Liver,  218 

bile  capillaries,  226 

capsule,  222 

connective  tissue,  222 

development,  218 

ducts,  228 

hepatic  cells,  224 

ligaments,  219 

lobules,  223 

perivascular  tissue,  227 

secretory  units,  229 

sinusoids,  125,  220,  227 

veins,  220 
Longitudinal  duct,  of  epoophoron,  286 
Lumen  of  glands,  36 
Lungs,  234 

alveoli,  240 

atria,  241 

development,  234,  236 

lobules,  236,  243 

nerves,  243 

pigment,  243 

pleura,  235,  242 

respiratory  bronchioles,  240 

structural  units,  243 

vessels,  235,  243 
Lunula,  316 
Lutein  cells,  291 
Lymph,  15 

follicles,  154 

glands,  154 

function,  158 

nodes  {lymph  glands),  154 

sinus,  156 

vessels,  157 

nodules,  154 

solitary,  154 
aggregate,  156,  214 

vessels,  137 

development,  138 
injections  of,  414 
stomata,  140 
valves,  139 
Lymphocytes,  47,  146 


INDEX. 


429 


Lymphoid  tissue,  34,  154 
Lyons  blue,  411 

M. 

Macula  lutea,  362 

acustica,  380 
Malleus,  382 

Mallory's  connective  tissue  stain,  412 
Malpighian  corpuscles  {renal  corpuscles),  253 

corpuscles  {splenic  nodules),  160 
Mamillars'  bodies,  338 
Mammary  glands,  313,  328 
areola,  330 
development,  328 
Margarin  crystals,  44 
Marrow,  bone,  152 
Mast  cells,  47,  148 
Meckel's  diverticulum,  195 
Mediastinum  of  the  ovary,  288 
of  the  testis,  275 
of  the  thorax,  235 
Medulla,  156 

oblongata,  335,  339 
spinalis  {spinal  cord),  114 
Medullary  groove,  18,  91 
plate,  91 
tube,  20,  91 
Medullated  nerve  fibers,  loi 
Megakaryocjle,  152 
Megaloblasts,  141 

Meibomian  glands  {tarsal  glands),  375 
Meissner's  corpuscles  {tactile  corpuscles),  105 

plexus  {submucous  plexus),  215 
Melanin,  45 
Membrana  basUaris,  of  the  cochlea,  387 

limitans  externa,  of  the  retina,  357,  35S 
propria,  in  general,  30 
limitans  interna,  of  the  retina,  359 
vestibularis,  of  the  cochlea,  385 
Membrane,  bone,  61 

Bowman's  (anterior  basal  membrane  of 

the  cornea),  369 
Descemet's  (posterior  basal  membrane 

of  the  cornea),  371 
hyaloid,  366 
pupillary,  355 

Reissner's  {membrana  vestibularis),    385 
tympanic,  382,  393 
Meninges,  351 
Menstruation,  298 
Mesencephalon,  334,  336 
Mesenchyma,  23,  25 
Mesenchymal  epithelium,  28 

tissues,  38 
Mesenter}',  211 
Mesoderm,  19 
Mesodermic  segments,  22 
Mesonephros,  244 
Mesothelium,  25,  27 
Mesovarium,  288 
Metaphase,  11 
Metencephalon,  335 
Methyl  green,  401 
Methylene  blue,  401 

and  eosine,  41 1 


Micrometer,  418 
Micron,  7 
•Microscope,  415 
Microsome,  2 
Microtome,  402 
Mid-brain,  334 
Milk,  329 
Mitome,  3 
Mitosis,  9 

anaphase  of,  12 

atypical,  14, 

heterotypical,  12,  270 

metaphase  of,  1 1 

prophase  of,  9 
Mixed  glands,  190 
Modiolus,  380 

MoU,  glands  of  {ciliary  glands),  375 
Monaster,  11 

Mononuclear  leucocytes,  146 
Monospireme,  10 

Montgomery's  glands  {areolar  glands),  330 
Morgagni,  hydatid  of  {appendix  testis),  280 

sinus  of,  {ventricle  of  larynx),  237 
Morula,  18 
Motor  cells,  91,  120 

nerves,  90 

endings,  107 
Mounting  sections,  408 
Mouth,  184 

development  of,  165 
Mucins,  40 
Mucoids,  40 
Mucous  bursae,  50 

glands,  33,  185 

tissue,  39 
Mucus,  33 

Miillerian  duct,  263,  285 
Miiller's  fibers  (of  the  retina),  358 

preserving  fluid,  404 
Muscle  tissue,  25,  77 

cardiac,  80 

comparison  of  the  three  t}^es,  85 

contraction,  82 

involuntary,  77 

smooth,  77 

striated,  85 

voluntary,  77 
Myenteric  plexus,  95,  198,  215 


N. 

Naboth,  ovules  of,  298 
Nails,  316 
Nasal  cavity,  357 

septum,  395 
Nasolachr}^mal  ducts,  378 
Nasmyth's  membrane  {cuticula  dentis) ,  73 
Nephrotome,  22 
Nerve  cells,  90 

commissural,  91 

motor,  91 

sensor}',  90 
Nerve  corpuscles,  105 
Nerve  endings,  102 
free,  102 


43° 


INDEX. 


Nerve  endings,  motor,  107 
motor  plates,  108 
sensory,  102 
tactile  menisci,  102 
Xer\'e  fibers,  90,  97 
afferent,  90 
axis  cylinders,  98 
commissural,  98 
motor,  90 
neurolemma  of,  98 
non-meduUated,  98 
of  central  nervous  system,  122 
reflex  path,  94 
Remak's,  98 
sensory,  90 
sheaths  of,  98 
fibrils,  97 
plexus,  94 
tissue,  25,  90 
development  of,  91 
Nerves,  10 1 

Nervous  system,  central,  91 
peripheral,  91 
sympathetic,  94 
Neumann's  membrane,  73 
Neural  groove,  91 
Neuraxon,  93,  94 
Neuroblasts,  115 
Neuroepithelium,  31 
Neuroglia  cells,  115 
Neurokeratin,  99 
Neurolemma,  98 
Neurone  theory,  123 
Neuroplasm,  97 
Neutrophiles,  148 
Nissl's  bodies,  120 
Nodes  of  Ranvier,  99 
Nodules,  aggregate,  156,  214 

solitary,  154 
Nodulus  thymicus,  167 
Normoblasts,  141 
Nose,  395 

development,  395 
nerves,  399 
olfactory  glands,  398 
vessels,  399 
vestibule,  395 
Notochord,  19,  21 
Nucleolus,  5 
Nucleoplasm,  2 
Nucleus  of  cells,  4 

of  the  nervous  system,  117 
Nuel's  spaces,  388 


O. 

Oculomotor  nerve,  96,  338 
Odontoblasts,  73 
Oesophagus,  196 
Odoriferous  glands,  326 
Oils  for  clearing  sections,  409 
Olfactory  bulb,  338,  349 

cells,  397 

epithelium,  397 

glands,  398 


Olive,  340 
Omental  bursa,  221 
Optic  cup,  353 

nerve,  354,  364 

recess,  334 

stalk,  353 

vesicle,  20,  334,  353 
Ooc}^es,  292 
Oogenesis,  292 
Oogonia,  292 
Ora  serrata,  355 
Oral  plate,  20,  165 
Organ  of  Corti  {spiral  organ),  387 

of  Rosenmiiller  (epoophoron),  286 
Organs,  25 
Orth's  fluid,  404 
Osmic  acid,  413 
Ossification,  53,  61 
Osteoblast,  54 
Osteoclast,  57 
Otoconia,  384 
Otocyst,  378 
Otoliths,  384 
Ovan.-,  288 

development  of,  287 

follicles,  288 

vessels  and  nerves,  294 
Ovulation,  290 
Ovum,  mature,  290 


Pacchionian  bodies  {arachnoid  granulations), 

351. 
Pacinian  corpuscles  {lamellar  corpuscles),  107 
Palate  processes,  395 
Palatine  glands,  189 

tonsils,  167 
Pallium,  338 
Palpebrse,  375 
Pancreas,  230 

dorsal,  230 

islands,  231 

ventral,  230 
Paneth,  cells  of,  207 
Panniculus  adiposus,  314 
Papilla,  duodenal,  205,  230 

genital,  193 

of  hair,  317 

of  optic  nerve,  354 

renal,  250 
Pai)illae,  epidermal,  313 

of  the  tongue,  177 
Paradidymis,  280 
Paraffin,  imbedding  in,  406 
Paraganglia,  114 
Paramitome,  3 
Paranucleus,  4 

Parathyreoid  glands,  167,  175 
Parietal  cells,  200 
Paroophoron,  286 
Parotid  gland,  187 
Parovarium  {epoophoron),  286 
Pavement  epithelium,  28 
Peduncles  of  the  cerebrum,  336 
Pelvis  of  kidney,  249 


INDEX. 


431 


Penis,  266,  281 

Peptic  glands,  200 

Pericardium,  135 

Perichondral  bone,  61 

Perichondrium,  51 

Peridental  membrane   (alveolar  periosteum), 

77 
PerUymph  spaces,  381 
Perilymphatic  duct,  392 
Perineum,  193 
Perineurium,  loi 
Periosteal  bone,  61  • 

Periosteum,  56,  59 
Peritonaeum,  211 

Petit,  canal  of  {zonular  spaces),  364 
Peyer's  patches  {aggregate  nodules),  156,  214 
Phagocytes,  8 
Pharyngeal  recess,  167 

tonsil,  169 
Pharynx,  184 

development,  165 
Pia  mater,  118,  351 
Pigment  cells,  45 
Pinguecula,  377 
Pineal  body,  338,  350 
Pillar  cells  of  spiral  organ,  387 
Pinna  (auricle),  382 
Pituitary  body  (hypophysis),  349 
Placenta,  301,  305 
Plasma,  151 
Plasma  cells,  47 
Plasmodium,  6 
Plates,  blood,  150 
Pleura,  236,  242 
Pleural  villi,  243 
Plexus  annularis,  374 

Auerbach's,  215 

cardiac,  94,  137 

chorioid,  352 

coeliac,  95 

gangliosus  ciliaris,  374 

Meissner's,  215 

myenteric,  95,  198,  215 

myospermaticus,  278 

pulmonary,  243 

solar,  95 

submucous,  95,  215 
Plica  semilunaris  of  the  eyelid,  377 
Plicae  adiposae,  of  pleura,  243 

circulares  of  small  intestine,  205 

palmatae,  of  uterus,  296 

semilunares,  of  the  large  intestine,  217 

transversales,  of  rectum,  217 

villosae,  of  stomach,  198 
Polar  globule,  293 
Polarity  of  cells,  33 
Polymorphonuclear  leucocytes,  146 
Pons,  335,  341 
Porta  hepatis,  229 
Portal  vein,  220 
Praeputial  glands,  324 
Precartilage,  50 
Premyelocytes,  153 
Primary  bone,  61 

follicles,  287 
Primitive  streak,  18 


Prisms,  enamel,  70 
Processus  vaginalis,  267,  288 

vermiformis,  215 
Proliferation  islands  of  the  placenta,  305 
Pronephros,  248 
Prosencephalon,  334 
Prostate,  280 
Proteid  absorption,  279 
Protoplasm,  2 
Protovertebrae,  22 
Pseudostratified  epithelium,  28 
Pulmonary  arches,  234 

plexus,  243 

veins,  235 
Pulp,  of  teeth,  68,  74 
Pupil,  of  dilatator  muscle,  369 

sphincter  muscle,  369 
Purkinje's  cells,  344 

fibers,  85,  135 
Pyloric  glands,  200 
Pylorus,  198 
Pyramids  of  Ferrein  {pars  radiata),  251 

of  the  medulla  oblongata,  339 
Pyramidal  cells,  346 

tracts  {cerebrospinal) ,  122,  339,  342,  345 
Pyrenin,  5 


Radial  fibers  of  the  retina,  358 
Ranvier,  nodes  of,  77 
Raphe  of  the  scrotum,  266 

of  the  medulla  oblongata,  340 
Reconstructions,  418 
Rectum,  217 
Red  corpuscles,  140 

color,  143 

number,  144 

shape,  142 
Reduction  division,  14 
Reflex  path  of  spinal  cord,  94 
Reflexa  (decidua  capsular  is),  301 
Reissner's  membrane  {membrana  vestibu- 
laris), 385 
Remak's  fibers,  98 
Renal  columns,  250 

corpuscles,  253 

pelvis,  260 

pyramids,  250 

tubules,  250 
Respiratory  bronchioles,  240 

tract,  234 
Restiform  body,  336,  340 
Resting  wandering  cells,  47 
Rete  Malpighii  {stratum  germinativum) ,  315 

testis,  265 
Reticular  tissue,  38 
Reticulin,  39 
Retina,  357 

cones,  357 

development,  353  » 

fovea  centralis,  362 

ganglion  retinae,  359 

macula  lutea,  362 

pars  ciliaris,  355,  362 

pars  iridica,  355,  368 


432 


INDEX. 


Retina,  pars  optica,  355,  357 
pigment  layer,  353,  357 
rods,  357  _ 

Resorcin-fuchsin,  412 

Retzius,  cells  of,  346 
lines  of,  72 

Rhinencephalon,  338 

Rhombencephalon,  334 

Rhomboidal  sinus,  20 

Rosenmuller,  organ  of  (epoophoron), 

Ruffini's  terminal  cylinders,  104 


Saccus  endolymphaticus,  392 

Sarcolemma,  79 

Sarcomeres,  82 

Sarcoplasm,  79 

Scala  media  {cochlear  dud),  380,  385 

tympani,  381,  385 

vestibuli,  381,  385 
Schlemm,  canal  of  {sinus  venosns  sclerae),  373 
Schreger's  lines,  72 
Schwann's  sheath,  98 
Sclera,  355,  369 
Sclerotome,  86 
Scrotum,  266 

Sebaceous  glands,  312,  324 
Secretion,  32 

internal,  35 
Secretory  capillaries,  36 
Sectioning,  402 
Segmentation,  18 
Semicircular  ducts,  384 

development,  379 
Seminal  fluid,  272 

vesicles,  279 
Sensory  decussation,  340 

nerve  cells,  90 

fibers,  90 

endings,  102 
Septula  testis,  275 
Septum  transversum,  218 
Serotina  {decidua  basalis),  301 
Serous  glands,  32,  187 

Sertoli's  cells  (sustcntacular  cells  of  the  tes- 
tis), 268 
Serum,  150 
Sexual  cells,  267,  288 
Scharlach  R,  412 
Sharpey's  fibers,  56,  76 
Silver  nitrate,  414 
Simple  epithelium,  27 
Sinus,  "coronary,  134 

lactiferous,  330 

urogenital,  193,  265,  287 

venosus,  134 
sclerae,  373 
Sinuses,  blood,  in  haemolymph  glands,  159 

lymph,  156 

of  the  dura,  351 
Sinusoids,  125,  227,  247 
Skin,  312 

corium,  313 

epidermis,  315 


Skin,  hair,  317 

nails,  316 

nerves,  327 

sebaceous  glands,  324 

sweat  glands,  325 

vessels,  327 
Small  intestine,  203 

blood  vessels,  211 

duodenum,  204 

glands,  206 

ileum,  205 

jejunum,  205 

lymphatic  vessels,  213 

lymphoid  tissue,  213 

mesentery,  211 

nerves,  215 

villi,  204 
Solitary  nodules,  154 
Spermatic  cord,  278 
Spermatid,  270 
Spermatocytes,  270 
Spermatogenesis,  270 
Spermatogonia,  268 
Spermatozoon,  269 
Spermium,  270 
Spinal  cord,  114 

cell  bodies,  120 

central  canal,  117 

columns,  115 

commissures,  115,  117 

dorsal  median  septum,  116 

dorsal  nucleus,  117 

ependyma,  122 

fasciculi,  122 

funiculi,  122 

gray  substance,  119 

neuroglia,  115,  118,  122 

pia  mater,  118 

substantia  gelatinosa,  118 

sulci,  116 

ventral  median  fissure,  116 

white  substance,  118 

zona  spongiosa,  118 

zona  terminalis,  118 
Spinal  ganglia,  92,  109,  no 

nerves,  93 
Spiracle,  165 
Spiral  organ,  380,  386 
Spireme,  12 
Splanchnic  nerves,  95 
Spleen,  159 

capsule,  163 

nerves,  163 

nodules,  160,  163 

pulp,  160,  162 
Spongioplasm,  3 
Squamous  cells,  29 
Stains,  general,  410 

Special,  411 
Staining  of  celloidin  sections,  409 

of  paraffin  sections,  408 
Stapes,  382 

Stenson's  duct  {parotid  duct),  187 
Stomach,  198 

glands,  200 
Stratified  epithelium,  29 


INDEX. 


433 


Striated  muscle  (cardiac,  80),  voluntary,  85 

Stroma  ovarii,  288 

Subarachnoid  space,  351 

Subcardinal  veins,  220 

Subcutaneous  tissue,  313 

Subdural  space,  351 

Sublingual  glands,  190 

Submaxillary  glands,  191 

Substantia  adamantina,  67 

alba,  118 

eburnea,  67 

gelatinosa,  118 

grisea,  119 

lentis,  365 

ossea,  67 
Suprarenal  gland,  33 1 
Sustentacular  cells,  of  ear,  38.1,  388 
of  nose,  397 
of  taste  buds,  179 
of  testis,  267 
Sweat  glands,  312,  325 
Sylvius,  aqueduct  of,  334 
Sympathetic  ganglia,  112 

nervous  system,  91 
Synapsis,  270 
Synarthrosis,  64 
Synchondrosis,  64 
Syncytium,  6 
Syndesmosis,  64 
Synovia,  67 

T. 

Tactile  cells,  103 

menisci,  102 
Taeniae,  217 
Tapetum  cellulosum,  367 

fibrosum,  367 
Tarsal  glands,  375 
Taste  buds,  179 

cells,  181 
Teeth,  67 

cement,  76 

dentine,  73 

enamel,  69 

pulp,  74 
Telencephalon,  335,  338 
Tellyesnizcky's  fluid,  404 
Tendon,  48 

spindles,  103 
Tenon's  capsule  {inter jascial  space),  374 
Terminal  bars,  29 

corpuscles,  105 

menisci,  102 
Testis,  267 

atrophy,  275 

cells,  267 

connective  tissue,  274 

convoluted  tubules,  273 

descent  of,  267 

interstitial  cells,  275 

nerves,  276 

rete,  274 

vessels,  276 
Thalamus,  338 

Thebesius,  veins  of  {venae  minimae),  136 
Thoracic  duct,  148 


Thymus,  167,  169 
Thymic  corpuscles,  172 
Thyreoid  gland,  166,  173 
Tissues,  18 

examination  of  fresh,  400 
Tomes's  fibers  {dental  fibers),  73 

processes,  71 
Tongue,  176 
Tonsils,  lingual,  169 

palatine,  167 

pharyngeal,  169 
Top  plate,  30 
Trachea,  238 

Triangular  ligaments  of  the  liver,  219 
Trigeminus  nerve,  96,  335 
Trochlear  nerve,  96,  336 
Trophoblast,  300 
Trophospongium,  4 
Tympanic  cavity,  385,  392 

■membrane,  382,  393 
Tyson's  glands,  324 

U. 

Umbilical  cord,  301,  308 

vein,  220,  316 
Umbilicus,  193 
Unna's  methylene  blue,  41 1 
Urachus,  193 
Ureter,  260 
Urethra,  193' 

female,  262 

male,  265,  281 
Uriniferous  tubules,  254 
Urogenital  sinus,  193,  265,  287 
Uterine  tubes,  286,  294 
Uterus,  296 

menstruating,  298 

pregnant,  300 
Utriculus,  379,  384 

V. 

Vacuoles,  4 

Vagina,  311 

Vagus  nerve,  96,  335 

Valves,  of  the  heart,  135 

of  the  lymph  vessels,  139 

of  the  veins,  132 
Valvulae  conniventes  {circular  folds),  205 
Vas  deferens  (ductus  deferens),  265,  278 

prominens,  385 
Vasa  aberrantia  of  the  liver,  229 

vasorum,  130 
Vascular  tissue,  25,  124 
Vasomotor  nerves,  130 
Vegetative  cells  {sustentacular  cells),  2 68 
Vena  cava  inferior,  220,  247,  248 
Veins,  131 

cardinal,  220,  246 

portal,  220 

pulmonary,  235 

umbilical,  220,  316 

vitelline,  24,  220 
Ventricles,  of  the  brain,  91,  334,  335,  33S 

of  the  heart,  133 


434 


INDEX. 


Vermiform  process,  215 
Vesicular  follicles,  289 
Vestibule,  of  labyrinth,  ■ 

of  nose,  385 

of  vagina,  287 
Vibrissae,  395 
Villi,  amniotic,  311 

chorionic,  300,  305 

pleural,  243 

intestinal,  241 

synovial,  67 
Visual  cells,  357 
Vitreous  body,  354,  366 

humor,  366 
Volkmann's  canals,  56 


White  fibers,  41 

substance,  of  the  spinal  cord,  118 
Winslow's  foramen  {joramen  epiploicutn),  221 
Wolffian  body,  244 

duct,  245,  285 

tubules,  265,  286 


Xylol,  409 


Yolk  sac,  193,  311 
stalk,  194,  310 


W. 

Wax   reconstructions,  419 
Weigert's  elastic  tissue  stain,  412 
Wharton's  duct  {submaxillary  duct),  iqi 

jelly    (mucous   tissue    of    the    umbilical 
cord),  39 
White  corpuscles,  145 


Z. 

Zenker's  fluid,  403 
Zona  pellucida,  293 
spongiosa,  118 
terminalis,  118 
Zonula  ciliaris,  362,  364 
Zymogen  granules,  201, 


233 


Vci"^ 


L^8 


